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HUMORAL (ANTIBODY-MEDIATED) IMMUNITY
Humoral immunity (immunité humorale/immunità umorale/inmunidad humoral) [ES], [F], is mediated by antibodies that are produced by B-cells and circulate in the blood and lymph streams. Antibodies specifically recognize a foreign antigen and mark it for destruction by other immune cells.
B-cells, one of the two major classes of lymphocytes, [I1],[I2],[E1],[E2],[F],originate and grow to maturity in the bone marrow before moving into the circulatory and lymph systems. During development, they differentiate in a number of sub-populations having diverse function but similar morphology. B-cells expose on their plasma membrane specific receptors molecules (immunoglobulin, Ig) that are able to recognize the antigen located on the surface of an invader. An immunocompetent but still immature B-lymphocyte is activated when the antigen binds to its matching receptor. The B cell engulfs and processes the antigen and displays a fragment, bound to a class II MHC protein, on the cell surface; the whole complex then binds to a helper T lymphocyte located nearby. This sensitizes (or primes) the B-cell which undergoes clonal selection, rapidly dividing by mitosis. Most of the clones become plasma cells and produce highly specific antibodies at a rate of more than 2000 molecules per second for the next few days. The antibodies produced by plasma cells are secreted into the blood, where they can bind to the specific antigen. The other B-cells become memory cells with a very long life and survive for many years in the body. When the organism is exposed for the first time to an antigen, the antibodies production starts slowly; if the same antigen is presented to the immune system again, the memory cells rapidly proliferate and differentiate into plasma cells to release a large amount of antibodies that eliminate the antigen.
Antibodies[I1],[I2],[E1],[E2],[E3], [F], [ES],comprise a large family of secreted glycoproteins, produced by plasma cells developing from primed B cells. Antibodies are able to recognize and bind specific antigens (in a lock-and-key fashion) forming an antigen-antibody complex; antigens are subsequently inactivated by tissue macrophages, NK cells or by the complement cascade proteins. There are five classes of antibodies: IgG (76% of total immunoglobulins), IgA (15%), IgM (8%), IgD (1%), and IgE (0.002%). IgM are mainly secreted in primary immune responses; they are also bound to the plasma membrane of B cells, acting as antigen receptors. IgG control the secondary immune responses and are the only antibodies that can cross the placental barrier to the fetus leading to immune protection of newborns conferred by the mother. IgE are responsible for autoimmune responses. Antibodies are Y-shaped molecules composed of two identical long polypeptides (Heavy or H chains) and two identical short polypeptides (Light or L chains) coupled together by disulfide bonds; some antibodies are also present in dimeric or pentameric forms. All chains contain a variable (V) and a constant (C) region. The variable region forms the antigen binding portion of the molecule; it is highly specific to target a given site (epitope) on the antigen, and the substitution of even a single amino acid residue severely modifies the antibody specificity, explaining the large variety of antigens that can be recognized by the immune system. The constant region interacts with macrophages or with the proteins of the complement system that attack antigen-receptor complexes.
Although antibodies can recognize an antigen and bind to it, they are not capable of destroying it without help: thus, some antibodies coat the foreign invaders to attract phagocytes, other combine with antigens to activate the complement proteins, that destroy and remove the invaders from the body. Still other types of antibodies block viruses from entering cells, or stimulate the functions of eosinophils and NK cells.
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