Antibody

An antibody (Ab), or immunoglobulin (Ig), is a large protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as those that exist on bacteria and virus cells, including those that cause disease. Each individual antibody recognizes one or more specific antigens, and antigens (a portmanteau of "antibody generator") of virtually any size and chemical composition can be recognized. Each of the branching chains comprising the "Y" of an antibody contains a paratope (the antigen-binding site) that specifically binds to one particular epitope (a specific part of an antigen bound by the paratope) on an antigen, allowing the two molecules to bind together with precision. Using this mechanism, antibodies can effectively "tag" the antigen (or a microbe or an infected cell bearing such an antigen) for attack by cells of the immune system, or can neutralize it directly (for example, by blocking a part of a virus that is essential for its ability to invade a host cell).

Antibodies may be borne on the surface of an immune cell, as in a B cell receptor, or they may exist freely by being secreted into the extracellular space. The term antibody generally refers to the free (secreted) form, while the term immunoglobulin can refer to either forms. Since they are, broadly speaking, the same protein, the terms are often treated as synonymous.

To allow the immune system to recognize millions of different antigens, the antigen-binding paratopes at each tip of the antibody come in an equally wide variety. The rest of an antibody's structure is much less variable; in humans, antibodies occur in five classes or isotypes: IgA, IgD, IgE, IgG, and IgM. Human IgG and IgA antibodies are also divided into discrete subclasses (IgG1, IgG2, IgG3, and IgG4; IgA1 and IgA2). The class refers to the functions triggered by the antibody (also known as effector functions), in addition to some other structural features.