A component of acquired immunity (as opposed to innate immunity), antibodies bind to the surfaces of specific bacteria, viruses, and other antigens (such as allergens). Once bound, they can trigger numerous responses from the rest of the immune system, protecting the body and helping it to get rid of the invader. There are five types of antibodies:
- IgM - The first response antibody. IgM is produced most strongly after a person's initial exposure to a new invader.
- IgG - The high affinity (tightly bonding) antibody type that is found in the blood and extracellular fluid. IgG helps encourage phagocytic cells to eat pathogens and can activate the complement system. It is also able to cross the placenta and provide passive immunity to a fetus.
- IgA - Found primarily in bodily secretions of the respiratory tract, gut and urogenital tract and is responsible for protecting the body's surfaces. IgA generally neutralizes pathogens directly.
- IgE - Triggers allergic reactions; also useful for fighting certain types of parasitic infections.
- IgD - Affects the development of certain immune cells.