Immunoglobulins (antibodies) are produced by
(or , a type of ), which are designed to
control the immune response in
extracellular fluids by binding to substances in the body that are recognized as foreign
(often proteins on the surface of and ).
Each Ig unit is made up of two heavy chains and two light
chains and has two antigen-binding sites.
Antibodies are diverse, with more than 1010 possible variations, yet each antibody
is designed to recognize only a specfic . Initially bound to B-cells,
upon encountering its
specific antigen, an stimulates the B-cell to produce copies of the antibody
with the aid of . The new antibodies, which are all designed to recognize the infecting
antigen, are released into the intercellular fluid where they bind to the infecting antigen, identifying it
for destruction by phagocytes and the complement system.
Immunoglobulins also play a central role in allergies when they bind to antigens that are not necessarily a threat to health and provoke an inflammatory reaction. There are five main types of antibody : ; ; ; and
, of which IgA, IgG, and IgM are the most common.