Antibody (Ab, aka immunoglobulin, Ig) is a large, Y-shaped protein, produced mainly by plasma cells (i.e. WBC's which secrete large volumes of antibodies), that is used by the immune system to identify and neutralize pathogens, e.g. bacteria and viruses.

• The antibody recognizes a unique molecule of the harmful agent, called an antigen, via the variable region
• Each tip of the "Y" of an antibody contains a paratope (analogous to a lack), that is specific for 1 particular epitope (similarly analogous to a key) on an antigen, allowing these 2 structures to bind together with precision
• Using this binding mechanism, an antibody can tag a microbe or an infected cell, for attack by other parts of the immune system, or can neutralize its target directly (e.g. by blocking a part of a microbe that is essential for its invasion and survival)
• The ability of an antibody to communicate with the other components of the immune system is mediated via its Fc region (located at the base of the "Y", which contains a conserved glycosylation site involved in these interactions
• The production of antibodies is the main function of the humoral immune system

Antibodies come in various isotypes or classes, the different suffixes of the antibody isotypes denoting the different types of heavy chains the antibody contains. This includes:

• IgA, found in mucosal areas, e.g. the gut, respiratory tract and urogenital tract, and prevents colonization by pathogens. It is also found in saliva, tears, and breast milk
• IgD, which functions mainly as an antigen receptor on B cells that have not been eposed to antigens. it has been shown to activate basophils and mast cells to produce antimicrobial factors
• IgE, which binds to allergens and triggers histamine release from mast cells and basophils, and is involved in allergy. It also protects against parasitic worms
• IgG, which is found in its 4 forms, providing the majority of antibody-based immunity against invading pathogens. it is the only antibody capable of crossing the placenta to give passive immunity to the fetus. It indicates a patient's IMMUNE STATUS to particular pathogens
• IgM, which is expressed on the surface of B cells (monomer), and in a secreted form (pentamer) with very high avidity. It eliminates pathogens in the early stages of B cell-mediated (humoral) immunity before there is sufficient IgG. It thus apears EARLY in the course of an infection, and usually reappear, to a lesser extent, after further exposure. It does NOT pass across the human placenta (only isotype IgG does). Thus, if found in a patient's blood, it indicates RECENT infection, or in a neonate's blood indicates INTRAUTERINE infection

• IVIG