The immunoglobulin superfamily (IgSF) is a large protein superfamily of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells. Molecules are categorized as members of this superfamily based on shared structural features with immunoglobulins (also known as antibodies); they all possess a domain known as an immunoglobulin domain or fold. Members of the IgSF include cell surface antigen receptors, co-receptors and co-stimulatory molecules of the immune system, molecules involved in antigen presentation to lymphocytes, cell adhesion molecules, certain cytokine receptors and intracellular muscle proteins. They are commonly associated with roles in the immune system. Otherwise, the sperm-specific protein IZUMO1, a member of the immunoglobulin superfamily, has also been identified as the only sperm membrane protein essential for sperm-egg fusion.
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Immunoglobulin superfamily in the context of Antibodies
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 bacteria and viruses, 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.