Humoral Immunity
B Cells & Plasma Cells
The second half of acquired immunity results in the production and release of antibodies. Upon activation by Th2 cells, B-cells differentiate into either plasma cells or memory B-cells. It is the plasma cells that secrete antibodies into the circulatory system to target and destroy invading antigens.
B-cells bind to an antigen that is specifically recognized by its unique receptors and then endocytose the B-cell receptor-antigen complex. The antigen is digested and displayed on the surface of the B-lymphocyte and the B-cell is briefly an APC. Th2 helper T-cells activate the B-cell and the B-cell differentiates into a plasma cell, which secretes antibodies that are specific to the antigen. Some B-cells become memory cells which "remember" what the antigen looks like so that it can respond more rapidly the 2nd time the host encounters it.
Antibodies
Antibodies are highly specific, "Y"-shaped immune proteins produced by activated plasma cells. Antibodies have 4 primary functions: 1) neutralize (trap) the antigen; 2) agglutinate (round up) the antigen and immobilize them until macrophages can ingest them; 3) opsonize (coat) the antigen to stimulate macrophage-mediate phagocytosis; or 4) "mark" the antigen for destruction by complement protein binding (complement fixation). There are 5 classes of antibodies: IgM, IgG, IgA, IgE and IgD.
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Types of Immunity
Active Immunity
After activation, the immune system will "remember" a particular pathogen and mount a more rapid and aggressive response to that pathogen if exposed again. This is known as active immunity and is the result of memory B and T cells. Natural active immunity occurs when one is exposed to a pathogen naturally. For instance, exposure to the chickenpox virus as a child results in protection from chickenpox in adulthood. Artificial active immunity also activates the immune system and stimulates the production of memory cells, but exposure to the pathogen is deliberate, for instance, a vaccine.
Passive Immunity
Temporary protection from specific pathogens may also occur. If the lymphocytes of the immune system are not activated, usually because the pathogen or antigen is removed by exogenous antibodies, then memory cells are not made. Natural passive immunity is where antibodies are acquired by an infant through the placenta or breast milk. Artificial passive immunity introduces antibodies into an individual, but the introduction is deliberate, for instance anti-venom for snake or spider bites, rabies or tetanus exposure.
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