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It would be like having a computer with a battery powered wireless keyboard whose battery went dead. This was such groundbreaking news that the Canadian Cancer Society has actually begun endorsing the vitamin as a cancer-prevention therapy. Hopefully that has clarified things for you and, as was previously mentioned, if there is something that you aren't sure whether you need or not, it's usually safer to leave cookies enabled, in case it does interact with one of the features you use on our site. By promoting blood flow, repair, and a high metabolism , exercise should be able to boost your immune system to more effectively eliminate bacterial infections like gonorrhea. He was alive and in good health 3 years later. Current Topics in Microbiology and Immunology.

Central nervous and endocrine systems

Immune system

Like other disease-fighting mushrooms, reishi mushrooms are a type of fungus that grow outdoors. Today, manufacturers of reishi products use a processing technique where reishi is boiled multiple times at a high pressure, allowing the active ingredients to be extracted to form a tincture.

Thanks to the concentrated compounds that these mushrooms contain, potential reishi mushroom benefits include protection against tumor formation, improved liver function, better heart health, enhanced blood sugar control and a reduced risk of asthma, allergies and infection. In supplement form, reishi mushrooms typically contain little to no calories and only a small amount of dietary fiber and protein. In particular, reishi mushrooms are jam-packed with antioxidants and health-promoting compounds such as polysacchraides and triterpenes.

These powerful compounds have been linked to a number of health benefits and have been shown to account for many of the anti-inflammatory, anti-cancer and anti-diabetic properties of the reishi mushroom. Over the past several decades, dozens of different studies conducted in Japan, China, the U. In fact, most people report a quick improvement in their energy levels, mental focus and mood while also experiencing a reduction in aches, pains, allergies, digestive issues and infections.

The secret behind their healing potential? Recent findings suggest that reishi mushrooms can lower inflammation and increase the release of natural killer cells, which work to remove various types of mutated cells from the body. Some of the ways that reishi mushrooms work to promote better health include: Many forms of holistic medicine take advantage of the full scope of benefits of reishi mushroom and use it to treat a variety of ailments.

Reishi mushrooms are an especially common ingredient in Traditional Chinese Medicine and have been used as a staple for thousands of years. They are believed to nourish the heart, preserve liver health, slow aging and enhance vitality, stamina and strength. Researchers believe that one of the most beneficial components of the reshi mushroom are its polysaccharides, which are a water-soluble type of nutrient found in carbohydrate foods that are known to have anti-tumor abilities.

Polysaccharides, which also found in other beneficial plant foods like sweet potatoes or beets, are immune-modulating substances. They are one of the compounds that help reishi mushrooms protect the DNA and block cell mutations while preserving healthy cells in the body. Triterpene compounds seem to inhibit tumor formation and metastases by limiting the attachment of cancerous cells to endothelial cells.

Studies have turned up promising results on the link between the reishi mushroom and cancer prevention; it has been successfully used to help fight cancer of the breasts, ovaries, prostate, liver and lungs in in-vitro studies, sometimes in combination with other treatments. The liver is one of the most vital organs in the body and is responsible for aiding in detoxification and helping clean, process, store and circulate healthy blood and nutrients. The triterpenes found in the reishi mushroom may have blood pressure-lowering abilities as well as benefits for blood-clotting and cholesterol, likely because they help lower inflammation within blood vessels and arteries while also restoring hormonal balance.

Reishi mushrooms work as adaptogens, meaning they can help your body cope with stress more efficiently and help regulate hormone levels to optimize your health. Although research is currently limited to mostly animal models, some studies indicate that reishi mushroom extract could help normalize levels of certain receptor hormones, which may be beneficial in the treatment of cancer.

Other studies show that it can also protect and positively impact the endocrine system , which encompasses the glands throughout the body that are responsible for the production of hormones. Sustaining high levels of blood sugar can bring detrimental effects to overall health, causing symptoms like fatigue, unintentional weight loss and frequent urination.

Some research shows that reishi mushrooms may have anti-diabetic properties, helping to maintain normal blood sugar levels to prevent adverse side effects. For example, one review out of Taiwan showed that reishi mushrooms were able to decrease levels of both blood sugar and insulin in mice.

They also helped modify levels of certain enzymes involved in blood sugar control and improved the way that the body uses insulin to transport sugar from the bloodstream to the tissues to be used as fuel. One of the active ingredients of reshi mushrooms are triterpenes, a type of ganoderic acid that is tied to a reduction in allergies and histamine reactions associated with asthma.

Triterpenes are capable of lowering allergic reactions because of the way they affect the immune system, strengthen the digestive organs, protect the gut lining, lower inflammation, inhibit a histamine release, improve oxygen utilization and improve liver functions. Reishi mushrooms are considered a natural antiviral, antibacterial and antifungal substance thanks to the active compounds that they contain.

Are mushrooms good for you? Additionally, different mushroom varieties have distinct differences in the ways that they are used and enjoyed. Wondering where to buy reishi mushroom supplements?

If you have a green thumb, you can also grow the red reishi mushroom at home by purchasing reishi spawn online. Products from Japan are considered to be some of the purest and are usually cultivated using techniques that preserve the delicate compounds in reishi. For this reason, T-cell receptors were difficult to isolate in the laboratory and were not identified until T-cell receptors consist of two polypeptide chains.

The most common type of receptor is called alpha-beta because it is composed of two different chains, one called alpha and the other beta. A less common type is the gamma-delta receptor, which contains a different set of chains, one gamma and one delta. A typical T cell may have as many as 20, receptor molecules on its membrane surface, all of either the alpha-beta or gamma-delta type.

The T-cell receptor molecule is embedded in the membrane of the cell, and a portion of the molecule extends away from the cell surface into the area surrounding the cell. The chains each contain two folded domains, one constant and one variable, an arrangement similar to that of the chains of antibody molecules.

And, as is true of antibody structure, the variable domains of the chains form an antigen-binding site. However, the T-cell receptor has only one antigen-binding site, unlike the basic antibody molecule, which has two. Many similarities exist between the structures of antibodies and those of T-cell receptors.

Therefore, it is not surprising that the organization of genes that encode the T-cell receptor chains is similar to that of immunoglobulin genes.

Similarities also exist between the mechanisms B cells use to generate antibody diversity and those used by T cells to create T-cell diversity. These commonalities suggest that both systems evolved from a more primitive and simpler recognition system.

Despite the structural similarities, the receptors on T cells function differently from those on B cells. The functional difference underlies the different roles played by B and T cells in the immune system. B cells secrete antibodies to antigens in blood and other body fluids, but T cells cannot bind to free-floating antigens.

Instead they bind to fragments of foreign proteins that are displayed on the surface of body cells. Thus, once a virus succeeds in infecting a cell, it is removed from the reach of circulating antibodies only to become susceptible to the defense system of the T cell.

But how do fragments of a foreign substance come to be displayed on the surface of a body cell? First, the substance must enter the cell, which can happen through either phagocytosis or infection. Next, the invader is partially digested by the body cell, and one of its fragments is moved to the surface of the cell, where it becomes bound to a cell-surface protein. This cell-surface protein is the product of one of a group of molecules encoded by the genes of the major histocompatibility complex MHC.

In humans MHC proteins were first discovered on leukocytes white blood cells and therefore are often referred to as HLA human leukocyte antigens. For information on the genetic basis of the HLA, see human genetics. There are two major types of MHC molecules: Two main types of mature T cells— cytotoxic T cells and helper T cells —are known.

Some scientists hypothesize the existence of a third type of mature T cell called regulatory T cells. Cytotoxic T cells destroy body cells that pose a threat to the individual—namely, cancer cells and cells containing harmful microorganisms.

Helper T cells do not directly kill other cells but instead help activate other white blood cells lymphocytes and macrophages , primarily by secreting a variety of cytokines that mediate changes in other cells. The function of regulatory T cells is poorly understood. To carry out their roles, helper T cells recognize foreign antigens in association with class II MHC molecules on the surfaces of macrophages or B cells. Cytotoxic T cells and regulatory T cells generally recognize target cells bearing antigens associated with class I molecules.

Because they recognize the same class of MHC molecule, cytotoxic and regulatory T cells are often grouped together; however, populations of both types of cells associated with class II molecules have been reported. Cytotoxic T cells can bind to virtually any cell in the body that has been invaded by a pathogen. T cells have another receptor, or coreceptor, on their surface that binds to the MHC molecule and provides additional strength to the bond between the T cell and the target cell.

These accessory receptors add strength to the bond between the T cell and the target cell. The T-cell receptor is associated with a group of molecules called the CD3 complex, or simply CD3, which is also necessary for T-cell activation. These molecules are agents that help transduce, or convert, the extracellular binding of the antigen and receptor into internal cellular signals; thus, they are called signal transducers.

Similar signal transducing molecules are associated with B-cell receptors. When T-cell precursors leave the bone marrow on their way to mature in the thymus , they do not yet express receptors for antigens and thus are indifferent to stimulation by them.

Within the thymus the T cells multiply many times as they pass through a meshwork of thymus cells. In the course of multiplication they acquire antigen receptors and differentiate into helper or cytotoxic T cells. As mentioned in the previous section, these cell types, similar in appearance, can be distinguished by their function and by the presence of the special surface proteins, CD4 and CD8.

Most T cells that multiply in the thymus also die there. This seems wasteful until it is remembered that the random generation of different antigen receptors yields a large proportion of receptors that recognize self antigens—i.

Most such self-reactive T cells die before they leave the thymus, so that those T cells that do emerge are the ones capable of recognizing foreign antigens. These travel via the blood to the lymphoid tissues, where, if suitably stimulated, they can again multiply and take part in immune reactions. The generation of T cells in the thymus is an ongoing process in young animals. In humans large numbers of T cells are produced before birth, but production gradually slows down during adulthood and is much diminished in old age , by which time the thymus has become small and partly atrophied.

Cell-mediated immunity persists throughout life, however, because some of the T cells that have emerged from the thymus continue to divide and function for a very long time. B-cell precursors are continuously generated in the bone marrow throughout life, but, as with T-cell generation, the rate diminishes with age.

Unless they are stimulated to mature, the majority of B cells also die, although those that have matured can survive for a long time in the lymphoid tissues.

Consequently, there is a continuous supply of new B cells throughout life. Those with antigen receptors capable of recognizing self antigens tend to be eliminated, though less effectively than are self-reactive T cells.

As a result, some self-reactive cells are always present in the B-cell population, along with the majority that recognize foreign antigens. The reason the self-reactive B cells normally do no harm is explained in the following section.

In its lifetime a lymphocyte may or may not come into contact with the antigen it is capable of recognizing, but if it does it can be activated to multiply into a large number of identical cells, called a clone. Each member of the clone carries the same antigen receptor and hence has the same antigen specificity as the original lymphocyte. The process, called clonal selection , is one of the fundamental concepts of immunology. Two types of cells are produced by clonal selection— effector cells and memory cells.

Effector cells are the relatively short-lived activated cells that defend the body in an immune response. Effector B cells are called plasma cells and secrete antibodies, and activated T cells include cytotoxic T cells and helper T cells, which carry out cell-mediated responses.

The production of effector cells in response to first-time exposure to an antigen is called the primary immune response. Memory cells are also produced at this time, but they do not become active at this point.

However, if the organism is reexposed to the same antigen that stimulated their formation, the body mounts a second immune response that is led by these long-lasting memory cells, which then give rise to another population of identical effector and memory cells. This secondary mechanism is known as immunological memory , and it is responsible for the lifetime immunities to diseases such as measles that arise from childhood exposure to the causative pathogen.

Helper T cells do not directly kill infected cells, as cytotoxic T cells do. Instead they help activate cytotoxic T cells and macrophages to attack infected cells, or they stimulate B cells to secrete antibodies. Helper T cells become activated by interacting with antigen-presenting cells, such as macrophages.

Antigen-presenting cells ingest a microbe, partially degrade it, and export fragments of the microbe—i. A receptor on the surface of the helper T cell then binds to the MHC-antigen complex. But this event alone does not activate the helper T cell. Another signal is required, and it is provided in one of two ways: If the first signal and one of the second signals are received, the helper T cell becomes activated to proliferate and to stimulate the appropriate immune cell.

If only the first signal is received, the T cell may be rendered anergic—that is, unable to respond to antigen. A discussion of helper-T-cell activation is complicated by the fact that helper T cells are not a uniform group of cells but rather can be divided into two general subpopulations— T H 1 and T H 2 cells—that have significantly different chemistry and function.

These populations can be distinguished by the cytokines they secrete. The main role of the T H 1 cells is to stimulate cell-mediated responses those involving cytotoxic T cells and macrophages , while T H 2 cells primarily assist in stimulating B cells to make antibodies.

Once the initial steps of activation have occurred, helper T cells synthesize other proteins, such as signaling proteins and the cell-surface receptors to which the signaling proteins bind. These signaling molecules play a critical role not only in activating the particular helper T cell but also in determining the ultimate functional role and final differentiation state of that cell.

For example, the helper T cell produces and displays IL-2 receptors on its surface and also secretes IL-2 molecules, which bind to these receptors and stimulate the helper T cell to grow and divide.

The overall result of helper-T-cell activation is an increase in the number of helper T cells that recognize a specific foreign antigen, and several T-cell cytokines are produced. The cytokines have other consequences, one of which is that IL-2 allows cytotoxic or regulatory T cells that recognize the same antigen to become activated and to multiply.

Cytotoxic T cells, in turn, can attack and kill other cells that express the foreign antigen in association with class I MHC molecules, which—as explained above—are present on almost all cells. So, for example, cytotoxic T cells can attack target cells that express antigens made by viruses or bacteria growing within them.

Regulatory T cells may be similar to cytotoxic T cells, but they are detected by their ability to suppress the action of B cells or even of helper T cells perhaps by killing them. Regulatory T cells thus act to damp down the immune response and can sometimes predominate so as to suppress it completely.

A B cell becomes activated when its receptor recognizes an antigen and binds to it. In most cases, however, B-cell activation is dependent on a second factor mentioned above—stimulation by an activated helper T cell.

Once a helper T cell has been activated by an antigen, it becomes capable of activating a B cell that has already encountered the same antigen. Activation is carried out through a cell-to-cell interaction that occurs between a protein called the CD40 ligand , which appears on the surface of the activated helper T cells, and the CD40 protein on the B-cell surface. The helper T cell also secretes cytokines, which can interact with the B cell and provide additional stimulation.

Antigens that induce a response in this manner, which is the typical method of B-cell activation, are called T-dependent antigens. Most antigens are T-dependent. Some, however, are able to stimulate B cells without the help of T cells. The T-independent antigens are usually large polymers with repeating, identical antigenic determinants. Such polymers often make up the outer coats and long, tail-like flagella of bacteria. Immunologists think that the enormous concentration of identical T-independent antigens creates a strong enough stimulus without requiring additional stimulation from helper T cells.

Interaction with antigens causes B cells to multiply into clones of immunoglobulin-secreting cells. Then the B cells are stimulated by various cytokines to develop into the antibody-producing cells called plasma cells. Each plasma cell can secrete several thousand molecules of immunoglobulin every minute and continue to do so for several days.

A large amount of that particular antibody is released into the circulation. The initial burst of antibody production gradually decreases as the stimulus is removed e. The process just described takes place among the circulating B lymphocytes. The B cells that are called memory cells , however, encounter antigen in the germinal centres—compartments in the lymphoid tissues where few T cells are present—and are activated in a different way.

Memory cells, especially those with the most effective receptors, multiply extensively, but they do not secrete antibody. Instead, they remain in the tissues and the circulation for many months or even years. If, with the help of T cells, memory B cells encounter the activating antigen again, these B cells rapidly respond by dividing to form both activated cells that manufacture and release their specific antibody and another group of memory cells.

So, for example, if the antigen is microbial and an individual is reinfected by the microbe, the memory cells trigger a rapid rise in the level of protective antibodies and thus prevent the associated illness from taking hold.

Many pathogenic microorganisms and toxins can be rendered harmless by the simple attachment of antibodies. For example, some harmful bacteria , such as those that cause diphtheria and tetanus , release toxins that poison essential body cells.

Antibodies, especially IgG, that combine with such toxins neutralize them. Also susceptible to simple antibody attachment are the many infectious microbes—including all viruses and some bacteria and protozoans —that live within the body cells. These pathogens bear special molecules that they use to attach themselves to the host cells so that they can penetrate and invade them.

Antibodies can bind to these molecules to prevent invasion. Antibody attachment also can immobilize bacteria and protozoans that swim by means of whiplike flagella. In these instances antibodies protect simply by combining with the repeating protein units that make up these structures, although they do not kill or dispose of the microbes.

The actual destruction of microbes involves phagocytosis by granulocytes and macrophages, and this is greatly facilitated by the participation of the complement system. Complement is a term used to denote a group of more than 30 proteins that act in concert to enhance the actions of other defense mechanisms of the body. Complement proteins are produced by liver cells and, in many tissues, by macrophages.

Most of these proteins circulate in the blood and other body fluids in an inactive form. They become activated in sequential fashion; once the first protein in the pathway is turned on, the following complement proteins are called into action, with each protein turning on the next one in line.

The action of complement is nonspecific—i. In fact, complement proteins probably evolved before antibodies. Complement functions are similar among many species, and corresponding components from one species can carry out the same functions when introduced into another species. The complement system is ingenious in providing a way for antibodies, whatever their specificity, to produce the same biological effects when they combine with antigens. Originally immunologists thought that the complement system was initiated only by antigen-antibody complexes, but later evidence showed that other substances, such as the surface components of a microorganism alone, could trigger complement activation.

Thus, there are two complement activation pathways: The term alternative is something of a misnomer because this pathway almost certainly evolved before the classical pathway. The terminology reflects the order of discovery, not the evolutionary age of the pathways. The classical and alternative pathways are composed of different proteins in the first part of their cascades, but eventually both pathways converge to activate the same complement components, which destroy and eliminate invading pathogens.

The classical complement pathway is activated most effectively by IgM and the most abundant of the immunoglobulins, IgG.

But, for activation to occur, antibodies must be bound to antigens the antigen-antibody complex mentioned above. Free antibodies do not activate complement. To initiate the cascade, the first complement protein in the pathway, C1, must interact with a bound immunoglobulin.

Specifically, C1 interacts with the tail of the Y portion of the bound antibody molecule—i. Once bound to the antibody, C1 is cleaved , a process that activates C1 and allows it to split and activate the next complement component in the series.

This process is repeated on the following proteins in the pathway until the complement protein C3 —the most abundant and biologically the most important component of the complement system—is activated. The classical and alternative complement pathways converge here, at the cleavage of the C3 molecule, which, once split, produces C3a and the large active form of C3, the fragment called C3b.

The small protein fragments that are released during the activation of complement are potent pharmacological agents that help promote an inflammatory response by causing mast cells and basophils to release histamine , which increases the permeability of blood vessels , and by attracting granulocytes and monocytes.

Thus, when a microbe penetrates the body, if antibodies reactive with its surface are already present or if the microorganism activates complement without the help of antibodies, through the alternative complement pathway , the complete complement sequence may be activated and the microbe killed by damage to its outer membrane.

This mechanism is effective only with bacteria that lack protective coats and with certain large viruses, but it is nevertheless important. Persons who lack C3 and thus cannot complete the later steps in the complement sequence are vulnerable to repeated bacterial infections.

Clearly such a biologically important chain of reactions could do more harm than good if its effects were to spread beyond the site of antigen invasion. Fortunately, the active intermediates at each stage in the complement sequence become rapidly inactivated or destroyed by inhibitors if they fail to initiate the next step. With rare exceptions, this confines the activation to the place in the body where it is needed.

Some cells that bear antigen-antibody complexes do not attract complement; their antibody molecules are far apart on the cell surface or are of a class that does not readily activate the complement system e. Other cells have outer membranes that are so tough or can be repaired so quickly that the cells are impermeable to activated complement. Still others are so large that phagocytes cannot ingest them. Such cells, however, can be attacked by killer cells present in the blood and lymphoid tissues.

Killer cells, which may be either cytotoxic T cells or natural killer cells , have receptors that bind to the tail portion of the IgG antibody molecule the part that does not bind to antigen. Once bound, killer cells insert a protein called perforin into the target cell, causing it to swell and burst.

Killer cells do not harm bacteria, but they play a role in destroying body cells infected by viruses and some parasites. The protection conferred by IgA antibodies, which are transported to the surface of mucous-membrane-lined passages, is somewhat different. Complement activation is not involved; there are no complement proteins in the lining of the gut or the respiratory tract.

Here the available immune defense mechanism is primarily the action of IgA combining with microbes to prevent them from entering the cells of the lining. The bound microbes are then swept out of the body. IgA also appears to direct certain types of cell-mediated killing. IgE antibodies also invoke unique mechanisms. As stated earlier, most IgE molecules are bound to special receptors on mast cells and basophils. The chemicals released cause a sudden increase in permeability of the local blood vessels, the adhesion and activation of platelets blood cell fragments that trigger clotting , which release their own active agents, the contraction of smooth muscle in the gut or in the respiratory tubes, and the secretion of fluids—all of which tend to dislodge large multicellular parasites such as hookworms.

Eosinophil granulocytes and IgE together are particularly effective at destroying parasites such as the flatworms that cause schistosomiasis. Therefore, IgE antibodies—although they can be a nuisance when they react with otherwise harmless antigens—appear to have a special protective role against the larger parasites.

A newborn mammal has no opportunity to develop protective antibodies on its own, unless, as happens very rarely, it was infected while in the uterus. The placenta generally prevents the maternal lymphocytes from crossing into the uterus, where they would recognize the fetal tissues as foreign antigens and cause a reaction similar to the rejection of an incompatible organ transplant.

How this happens depends on the structure of the placenta, which varies among species. In humans maternal IgG antibodies—but not those of the other immunoglobulin classes—are transported across the placenta into the fetal bloodstream throughout the second two-thirds of pregnancy. In many rodents a similar transfer occurs, but primarily across the yolk sac.

In horses and cattle , which have more layers of cells in their placentas, no antibodies are transferred during fetal life, and the newborn arrives into the world with no components of specific immunity. There is, however, a second mechanism that makes up for this deficiency. The early milk colostrum is very rich in antibodies—mainly IgA but also some IgM and IgG—and during the first few days of life the newborn mammal can absorb these proteins intact from the digestive tract directly into the bloodstream.

Drinking colostrum is therefore essential for newborn horses and cattle and required to a somewhat lesser extent by other mammals.

The capacity of the digestive tract to absorb intact proteins must not last beyond one or two weeks, since once foods other than milk are ingested, the proteins and other antigens in them would also be absorbed intact and could act as immunogens to which the growing animal would become allergic see immune system disorder: IgA in milk is, however, rather resistant to digestion and can function within the gut even after intact absorption into the bloodstream has ended.

Human colostrum is also rich in IgA, with the concentration highest immediately after birth. After a newborn has received its supply of maternal antibodies, it is as fully protected as its mother. This means, of course, that if the mother has not developed immunity to a particular pathogen, the newborn will likewise be unprotected.

For this reason, a physician may recommend that a prospective mother receive immunizations against tetanus and certain other disorders. The active immunization of pregnant women against certain viral diseases, such as rubella [German measles], must be avoided, however, because the immunizing agent can cross the placenta and produce severe fetal complications.

As important as the passively transferred maternal antibodies are, their effects are only temporary. The maternal antibodies in the blood become diluted as the animal grows; moreover, they gradually succumb to normal metabolic breakdown.

Because the active development of acquired immunity is a slow and gradual process, young mammals actually become more susceptible to infection during their early stages of growth than they are immediately after birth. Occasionally the transfer of maternal antibodies during fetal life can have harmful consequences. The most severe form of erythroblastosis fetalis is Rh hemolytic disease , which develops when:. Rh hemolytic disease can be prevented by giving the mother injections of anti-Rh antibody shortly after the birth of an Rh-positive child.

In addition to their importance in cooperating with B cells that secrete specific antibodies, T cells have important, separate roles in protecting against antigens that have escaped or bypassed antibody defenses. Immunologists have long recognized that antibodies do not necessarily protect against viral infections, because many viruses can spread directly from cell to cell and thus avoid encountering antibodies in the bloodstream.

It is also known that persons who fail to make antibodies are very susceptible to bacterial infections but are not unduly liable to viral infections. Protection in these cases results from cell-mediated immunity, which destroys and disposes of body cells in which viruses or other intracellular parasites such as the bacteria that cause tuberculosis and leprosy are actively growing, thus depriving microorganisms of their place to grow and exposing them to antibodies.

As discussed in the section Activation of T and B lymphocytes , cell-mediated immunity has two mechanisms. One involves activated helper T cells , which release cytokines. In particular, the gamma interferon produced by helper T cells greatly increases the ability of macrophages to kill ingested microbes; this can tip the balance against microbes that otherwise resist killing. Gamma interferon also stimulates natural killer cells.

The second mechanism of cell-mediated immunity involves cytotoxic T cells. They attach themselves by their receptors to target cells whose surface expresses appropriate antigens notably ones made by developing viruses and damage the infected cells enough to kill them. Cytotoxic T cells may kill infected cells in a number of ways. The mechanism of killing used by a given cytotoxic T cell depends mainly on a number of costimulatory signals.

In short, cytotoxic T cells can kill their target cells either through the use of pore-forming molecules, such as perforins and various components of cytoplasmic granules, or by triggering a series of events with the target cell that activate a cell death program, a process called apoptosis. In general, the granular cytotoxic T cells tend to kill cells directly by releasing the potent contents of their cytotoxic granules at the site of cell-to-cell contact.

This renders the cell membrane of the target cell permeable, which allows the cellular contents to leak out and the cell to die.

The nongranular cytotoxic T cells often kill cells by inducing apoptosis, usually through the activation of a cell-surface protein called Fas. When a protein on the surface of the cytotoxic T cell interacts with the Fas protein on the target cell, Fas is activated and sends a signal to the nucleus of the target cell, thus initiating the cell death process.

The target cell essentially commits suicide, thereby destroying the virus within the cell as well. Cancer cells are normal body cells that have been altered in a manner that allows them to divide relentlessly, ignoring normal signals of restraint. As a result, cancer cells form clusters of cells, called tumours , that invade and colonize tissues, eventually undermining organ function and causing death. In the early 20th century the pioneering immunologist Paul Ehrlich pointed out that the enormous multiplication and differentiation of cells during prenatal life must afford many opportunities for aberrant cells to appear and grow but that immune mechanisms eliminate such cells.

Although it has its compelling aspects, the immunosurveillance theory remains just a theory, and a controversial one at that. The role of the immune system in protecting against cancer has not been fully explained, but nevertheless there is no question that in some instances the immune system can distinguish cancer cells from normal cells.

The study of tumour immunology has shown unequivocally that cancer cells do carry antigens that are not present on healthy cells. Immunologists distinguish broadly between two types of tumour antigens: In both cases these antigens have been shown to evoke an immune response, although not necessarily one strong enough to eliminate the tumour.

Why does a tumour continue to grow if an immune response against it is induced? Through animal experiments, a number of mechanisms have been identified that allow tumours to avoid recognition and destruction by the immune system:. Other dysfunctions of the immune system, such as immune suppression and immune deficiency, may contribute to cancer development and growth.

Individuals such as transplant patients who have been treated with immunosuppressive drugs for a long period of time are more likely to develop certain types of cancer, as are patients with immunodeficiency diseases.

For example, people with AIDS acquired immunodeficiency syndrome are more prone to developing cancers associated with viruses, such as Kaposi sarcoma. The incidence of cancer also increases greatly in old age, when some immune responses decline. But defective immune responses may not be the major factor involved in cancer development in the elderly, since genetic mutations that are linked to cancer also accumulate with age.

Much research has been devoted to developing effective immunotherapies against cancer. One avenue of research has focused on finding ways to immunize patients against the specific cancer growing within them. This approach targets tumour-specific antigens found on the cancer cells. However, investigators are working to develop vaccines that stimulate an immune response to these antigens, hoping that the reaction would be strong enough to eliminate the cancer. Prophylactic immunization refers to the artificial establishment of specific immunity, a technique that has significantly reduced suffering and death from a variety of infectious diseases.

There are two types of prophylactic immunization: It is sometimes the case that an infectious organism or a poisonous substance can have such a rapid deleterious effect that the victim does not have time to develop an immune response spontaneously.

At such times passive immunization with preformed antibodies can provide life-saving assistance in combating the pathogen or poison. This situation may arise in victims of poisonous snakebites or botulism , as well as in those in whom such infections as diphtheria , tetanus , or gas gangrene have progressed to the point at which bacterial toxins have been absorbed into the bloodstream. It is also the case with bites from a rabid animal, although active immunization is begun at the same time, since the spread of the rabies infection to the central nervous system is relatively slow.

Physicians use passive immunization as temporary protection for persons traveling to countries where hepatitis B is prevalent. Passive immunization provides antibodies to persons who suffer from B-cell deficiencies and are therefore unable to make antibodies for themselves see immune system disorder: Also, as discussed earlier, passive immunizations of anti-Rh antibody can prevent erythroblastosis fetalis. Protective immunoglobulins—primarily of the IgG class—can be prepared from the blood of humans or other species e.

These preparations are known as antiserums. This explains the original term for passive immunization, which is serum therapy. Human antiserum is used whenever it is available, because IgG from other species is far more likely to provoke an immune response that will eliminate the antibody and may lead to serum sickness see immune system disorder: Active immunization aims to ensure that a sufficient supply of antibodies or T and B cells that react against a potential infectious agent or toxin are present in the body before infection occurs or the toxin is encountered.

Once it has been primed, the immune system either can prevent the pathogen from establishing itself or can rapidly mobilize the various protective mechanisms described above to abort the infection or toxin in its earliest stages.

The vaccines used to provide active immunization need not contain living microbes. What matters is that they include the antigens important in evoking a protective response and that those antigens be administered in a harmless form sufficient in amount and persistence to produce an immune response similar to the natural infection.

Bacterial toxins, such as those that cause tetanus or diphtheria, can be rendered harmless by treatment with formaldehyde without affecting their ability to act as immunogens. These modified toxins, or toxoids , usually are adsorbed onto an inorganic gel before being administered, an approach that increases the likelihood that the toxoid will be retained in a macrophage. Toxoids elicit effective, long-lasting immunity against bacterial toxins.

When immunization against several antigenic determinants is desired or the important antigenic component is not known, it may be prudent to use the entire microbe, which has been killed in a manner that does not alter it significantly. In other cases, researchers have developed attenuated i. Attenuated vaccines cause an infection but do not produce the full array of signs and symptoms of the disease, because the infectious agent multiplies to only a limited extent in the body and never reverts to the virulent form.

The use of such live microbes provides the most effective prophylaxis of all, since they truly imitate a mild form of the natural infection. Such are the vaccines for yellow fever , poliomyelitis oral vaccine , measles , rubella , and tuberculosis. Although sufficiently attenuated as far as healthy persons are concerned, live vaccines may cause the full disease in persons who have an immune deficiency. Most vaccines are administered by injection, but a few are given orally.

Ultimately mucosal vaccines those administered to mucosal surfaces such as those lining the gut, nasal passages, or the urogenital tract may be the most effective vaccines available because of their unique ability to stimulate IgA responses and because of their ease of administration. Recombinant DNA technology has allowed researchers to use modified bacteria and viruses that are not harmful to humans to immunize individuals against an antigen from a pathogenic microorganism.

This approach involves introducing into the DNA of the harmless microorganism a gene from a pathogenic organism that encodes an antigen capable of eliciting a protective immune response but not the full-blown disease. Once inoculated into the host, the microorganism generates the protective antigen of the pathogen and immunizes the host. An effective oral vaccine against cholera was developed based on this approach. Sometimes different strains of a microorganism, each characterized by a different antigenic determinant, give rise to the same disease.

In such cases neither natural infection nor prophylactic immunization with any one strain protects against infection by the others. For example, a variety of virus strains cause the common cold , but it is impractical to immunize against each strain. On the other hand, although there are more than 60 different strains of pneumococci that can cause bacterial pneumonia , some strains are much more common than others.

Consequently a vaccine containing antigens from up to 14 of the most common strains is useful in protecting persons at special risk. Active immunization is often the most effective and least costly method of protecting against an infectious disease. Vaccination campaigns against many diseases, such as diphtheria, polio, and measles, have been tremendously successful.

In cases in which 95 percent or more of the population at risk is protected and humans are the only reservoir of infection, active immunization can lead to the worldwide eradication of the infectious agent, as has been achieved in the case of smallpox. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.

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Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article. Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Humphrey Samuel Scott Perdue. It brings about lysis bursting of the target cell by activating subsequent steps in the cascade, leading to the formation of a ringlike structure called the membrane attack complex.

This structure, which is composed of complement proteins C5 through C9, inserts itself into the membrane of the invading pathogen and creates a hole through which the cell contents leak out, killing the cell. But perhaps the most important result of C3b production is that great numbers of C3b molecules are deposited on the surface of an invading pathogen in a process called opsonization.

This makes the microorganism more attractive to phagocytic cells such as macrophages and neutrophils. The attraction occurs because receptors on the surface of phagocytes recognize and bind to the C3b molecule on the surface of the pathogen, stimulating phagocytosis. The microbe is then killed by digestive enzymes present in the phagocytes. If microbes are not immediately killed and are able to reach the bloodstream or the liver, spleen, or bone marrow, they can become coated with antibody and complement there and be ingested by phagocytes.

The fetus is Rh-positive; that is, its red blood cells carry an antigen known as the Rh factor.

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