Immunoglobulin family

A group of functionally and structurally related proteins that play a key role in the immune system is called the large immunoglobulin family (immunoglobulin superfamily). The immunoglobulins include antibodies, specific receptors for T-lymphocyte and B-lymphocytes, HLA molecules, adhesion molecules, and growth factor receptors. They are expressed mainly on leukocytes, but some are also found on other cells (e.g. HLA class I on all nuclear cells). They occur as free molecules or are incorporated into the cell membrane.

Immunoglobulin domain
A characteristic feature of immunoglobulins is the presence of the so-called immunoglobulin domain in their structure. The domains contain about 100 amino acids. They have a globular structure. The peculiarity is that they form incomplete rings connected by a sulphide bridge of two cysteines (see figure). Immunoglobulin domains are classified according to their variability within a single type of molecule.

Variable
Domains with high variability (hypervariable). They are usually found in the part of the molecule that contacts the antigens or ligands. They are referred to as V 1-n.

Constant
This type of immunoglobulin domain is characteristic of single molecules. They further determine the reaction upon ligand binding. They are referred to as C 1-n.

Transient
Domains that retain a partially constant form but may vary are called transient. They are denoted H1-n.

Function of immunoglobulins
All immunoglobulin molecules are able to recognize their specific ligands (with varying degrees of accuracy). They are therefore essential agents in specific immunity.

Related articles

 * Antibody
 * Specific immunity