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WHAT IS THE ROLE OF IMMUNOGLOBULINS IN THE IMMUNE RESPONSE? |
The main function of immunoglobulins is to react against antigens in order to mediate their elimination. |
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Immunoglobulins react with antigens in two different ways:
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The main biological functions of immunoglobulins are: |
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Complement activation. The complement system is an unspecific defence mechanism of the immune system (innate immunity). It has a role in a large number of inflammatory and cytotoxic reactions and macrophage activation. It is activated by several mechanisms (classical pathway, alternative pathway, chapter 7). Some immunoglobulins can activate the complement by the classical pathway. This type of activation is performed mainly by IgM, followed by IgG by means of their Fc fragments. Antibodies bound to the infected cell membrane or to bacteria trigger a cytotoxic reaction, which is very effective and able to destroy the cell membrane. The alternative pathway (chapter 7) is activated by IgG and IgA. |
Agglutination. Agglutination of bacteria and viruses is another biological activity of the immunoglobulins, above all of the IgM. Antibody dependent cellular cytotoxiciy (ADCC). Cytotoxic phenomena are not only induced by T CD 8+ lymphocytes; other immune cells, such as macrophages or NK cells, can also destroy cells with the cooperation of antibodies. This process happens when an antibody, usually IgG and sometimes also IgE, recognizes an antigen in the membrane of a cell, and then reacts with it, engulfing it (a process similar to opsonization);. The Fc fragment remains free. Those cells with cytotoxic activity and Fc receptors, such as NK cells and macrophages, bind the Fc fragment of the immunoglobulin and then trigger the citotoxicity activity. In this case, cytotoxicity is cell induced, but the reaction specifity will depend on the antibody. Both the Fab fragment (for antigen binding the membrane) and Fc fragment (for effector cell binding: NK or macrophage) act in this process. |
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Neutralization. Some immunoglobulin isotypes, such as IgG, IgM and IgA, are able to bind toxins, bacteria or viruses and neutralize their activity. In the case of viruses, the neutralization process allows antibodies to prevent viruses from infecting cells, due to the fact that antibodies coat the virus fragment needed for it to bind to the cell. In this case, only the Fab fragment acts. |
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Opsonization. This
is the phenomenom by which the antibodies
enwrap an antigen (viruses, bacteria, etc), and activate phagocytosis by means of Fc fragments of
macrophages, neutrophils or polymorphonuclear leukocytes.
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Mucosa protection. IgA protects the epithelial surfaces from infectious agents. Its dimer or tetramer conformation allows having four to eight antigen binding sites. This make IgA tremendously effective against different bacterial antigens, by means of ADCC reactions (IgA is not bactericidal). IgA has a great capability to neutralize some viruses. |
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