noun, plural: immunoglobulins
(1) Any of several classes of structurally related glycoproteins in the blood serum, produced by plasma cells as an immune response against foreign agents (antigens); antibody
(2) The fraction of the blood serum containing antibodies
An immunoglobulin (Ig) is a glycoprotein; it is comprised of a glycan and a protein constituent. The basic functional unit of an immunoglobulin has a Y-shape. It consists of two identical heavy chains and two identical light chains. In each of these chains, there are variable and constant regions. The apical portion of the variable regions serves as antigen-binding site. The plasma cell produces immunoglobulins as a response to an antigen, i.e. non-self foreign agent.
The immunoglobulin may also be described in this way: it is composed of two Fab (fragment, antigen-binding) regions and one fc (fragment, crystallizable) region. The Fabs include the antigen-combining sites whereas the fc region consists of the remaining constant sequence domains of the heavy chains, cell-binding site, and complement-binding sites.
In placental mammals (including humans), there are five classes (or isotypes) of immunoglobulins based on structure and biological activity:
- immunoglobulin A (IgA) – a dimer with α (alpha) heavy chain
- immunoglobulin D (IgD) – a monomer with δ (delta) heavy chain
- immunoglobulin E (IgE) – a monomer with ε (epsilon) heavy chain
- immunoglobulin G (IgG) – a monomer with γ (gamma) heavy chain
- immunoglobulin M (IgM) – a pentamer with μ (mu) heavy chain
Immunoglobulin and antibody are two terms that are used interchangeably. Immunoglobulin got its name, though, from immuno-, as being associated with the immune system, and globulin, which refers to any of a group of simple proteins insoluble in pure water but dissolves in dilute salt solutions. There are four classes of globulins (i.e. alpha 1, alpha 2, beta, and gamma). Immunoglobulins (although not all) belong to the gamma globulins. The immunoglobulins make up the most of gamma globulin fraction of the blood proteins.
Word origin: immuno– (relating to immune) + globulin.