Immunoglobulins (antibodies) are proteins produced by plasma cells (or B-Cells , a type of lymphocyte ), which are designed to control the immune response in extracellular fluids by binding to substances in the body that are recognized as foreign antigens (often proteins on the surface of bacteria and viruses ).
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 antigen . Initially bound to B-cells, upon encountering its specific antigen, an antibody/antigen complex stimulates the B-cell to produce copies of the antibody with the aid of helper T-cells . 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 : IgA ; IgD ; IgE ; IgG and IgM , of which IgA, IgG, and IgM are the most common.