2 -Microglobulin--a free immunoglobulin domain.

Analysis of the primary structure of beta(2)-microglobulin indicates that this human protein is homologous in sequence to the constant portion of immunoglobulin light chains (C(L)), and to the homology regions (C(H)1, C(H)2, and C(H)3) of the constant portion of gamma1 (heavy) chains of immunoglobulin G. Homology with the C(H)3 region is particularly striking. No convincing homology could be demonstrated by similar comparisons with the variable regions of immunoglobulin light and heavy chains. beta(2)-Microglobulin contains an intrachain disulfide loop of 57 amino-acid residues that is similar in size to disulfide loops found in the constant regions of immunoglobulin G. These findings suggest that beta(2)-microglobulin is a free immunoglobulin domain, possibly serving an effector function similar to that of the C(H)3 domain of gamma1 chains of immunoglobulin G.