(1) Homologous unit of approximately 110-120 amino acids, groups of which make up the light and heavy chains of the immunoglobulin molecule; each serves a specific function. The light chain has two domains, one in the variable region and one in the constant region of the chain; the heavy chain has four to five domains, depending on the class of immunoglobulin, one in the variable region and the remaining ones in the constant region. (2) A region of a protein having some distinctive physical feature or role. (3) An independently folded, globular structure composed of one section of a polypeptide chain. A domain may interact with another domain; it may be associated with a particular function. Domains can vary in size. [Fr. domaine, fr. L. dominium, property, dominion]