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  1. Innate vs. adaptive immunity
  2. Innate immunity
  3. Immune System (for Parents) - KidsHealth
  4. Immune system
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It is designed to identify and destroy harmful foreign organisms pathogens Pathogen Any disease-causing substance.

Most commonly used for organisms e. Source: wikipedia. The pathogens causing the vaccine-preventable diseases Vaccine-preventable diseases Diseases for which vaccines exist that can confer partial or complete protection. Some bacteria cause disease. A virus can replicate themselves only within cells of living hosts.

The immune system responds to bacteria and viruses in a very complex way: it recognizes unique molecules antigens Antigen A foreign substance in the body that triggers the production of antibodies.

Innate vs. adaptive immunity

During the primary immune response Immune response The body's defense against foreign objects or organisms, such as bacteria, viruses or transplanted organs or tissue. If a cell or tissue such as bacteria or a transplanted organ is recognized as not belonging to the body, the immune system will act against the "invader. These memory cells recognize antigens on the pathogens they have encountered before, triggering the immune system to respond faster and more effectively than on the first exposure.

Source 1: wikipedia.

The Cellular Immune Response

These cells allow the body to sample potential pathogens from the intestinal lumen. Dendritic cells then take the antigen to the regional lymph nodes, where an immune response is mounted. Figure 2. Both use M cells to transport antigen inside the body so that immune responses can be mounted.


Innate immunity

The body fights bacterial pathogens with a wide variety of immunological mechanisms, essentially trying to find one that is effective. Bacteria such as Mycobacterium leprae , the cause of leprosy, are resistant to lysosomal enzymes and can persist in macrophage organelles or escape into the cytosol. In such situations, infected macrophages receiving cytokine signals from Th1 cells turn on special metabolic pathways.

Macrophage oxidative metabolism is hostile to intracellular bacteria, often relying on the production of nitric oxide to kill the bacteria inside the macrophage. Fungal infections, such as those from Aspergillus , Candida , and Pneumocystis , are largely opportunistic infections that take advantage of suppressed immune responses.

Most of the same immune mechanisms effective against bacteria have similar effects on fungi, both of which have characteristic cell wall structures that protect their cells. Worm parasites such as helminths are seen as the primary reason why the mucosal immune response, IgE-mediated allergy and asthma, and eosinophils evolved.

Immune System (for Parents) - KidsHealth

These parasites were at one time very common in human society. When infecting a human, often via contaminated food, some worms take up residence in the gastrointestinal tract. Eosinophils are attracted to the site by T cell cytokines, which release their granule contents upon their arrival. Mast cell degranulation also occurs, and the fluid leakage caused by the increase in local vascular permeability is thought to have a flushing action on the parasite, expelling its larvae from the body. Furthermore, if IgE labels the parasite, the eosinophils can bind to it by its Fc receptor.

The primary mechanisms against viruses are NK cells, interferons, and cytotoxic T cells. Antibodies are effective against viruses mostly during protection, where an immune individual can neutralize them based on a previous exposure. Antibodies have no effect on viruses or other intracellular pathogens once they enter the cell, since antibodies are not able to penetrate the plasma membrane of the cell.

Immune system

Many cells respond to viral infections by downregulating their expression of MHC class I molecules. This is to the advantage of the virus, because without class I expression, cytotoxic T cells have no activity. NK cells, however, can recognize virally infected class I-negative cells and destroy them. Thus, NK and cytotoxic T cells have complementary activities against virally infected cells.

Interferons have activity in slowing viral replication and are used in the treatment of certain viral diseases, such as hepatitis B and C, but their ability to eliminate the virus completely is limited. The cytotoxic T cell response, though, is key, as it eventually overwhelms the virus and kills infected cells before the virus can complete its replicative cycle.

Clonal expansion and the ability of cytotoxic T cells to kill more than one target cell make these cells especially effective against viruses. In fact, without cytotoxic T cells, it is likely that humans would all die at some point from a viral infection if no vaccine were available. It is important to keep in mind that although the immune system has evolved to be able to control many pathogens, pathogens themselves have evolved ways to evade the immune response.

An example already mentioned is in Mycobactrium tuberculosis , which has evolved a complex cell wall that is resistant to the digestive enzymes of the macrophages that ingest them, and thus persists in the host, causing the chronic disease tuberculosis. But keep in mind, although it seems as if pathogens have a will of their own, they do not.

Bacteria sometimes evade immune responses because they exist in multiple strains, such as different groups of Staphylococcus aureus. One small group of strains of this bacterium, however, called methicillin-resistant Staphylococcus aureus , has become resistant to multiple antibiotics and is essentially untreatable. Different bacterial strains differ in the antigens on their surfaces. The immune response against one strain antigen does not affect the other; thus, the species survives. Another method of immune evasion is mutation.

New vaccine formulations must be derived for each flu season. Genetic recombination—the combining of gene segments from two different pathogens—is an efficient form of immune evasion. For example, the influenza virus contains gene segments that can recombine when two different viruses infect the same cell. Recombination between human and pig influenza viruses led to the H1N1 swine flu outbreak. Pathogens can produce immunosuppressive molecules that impair immune function, and there are several different types.

Viruses are especially good at evading the immune response in this way, and many types of viruses have been shown to suppress the host immune response in ways much more subtle than the wholesale destruction caused by HIV. Early childhood is a time when the body develops much of its immunological memory that protects it from diseases in adulthood.

The components of the immune response that have the maximum effectiveness against a pathogen are often associated with the class of pathogen involved. Bacteria and fungi are especially susceptible to damage by complement proteins, whereas viruses are taken care of by interferons and cytotoxic T cells. Worms are attacked by eosinophils.

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The immune system and pathogens are in a slow, evolutionary race to see who stays on top. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content.