<p>The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulphide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three (in alpha, delta and gamma) or four (in epsilon and mu) constant domains (CH1 to CH4). Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set (variable; <db_xref db="INTERPRO" dbkey="IPR013106"/>), C1-set (constant-1; <db_xref db="INTERPRO" dbkey="IPR003597"/>), C2-set (constant-2; <db_xref db="INTERPRO" dbkey="IPR008424"/>) and I-set (intermediate; <db_xref db="INTERPRO" dbkey="IPR013098"/>) [<cite idref="PUB00014840"/>]. Structural studies have shown that these domains share a common core Greek-key beta-sandwich structure, with the types differing in the number of strands in the beta-sheets as well as in their sequence patterns [<cite idref="PUB00015110"/>, <cite idref="PUB00010610"/>].</p><p>Immunoglobulin-like domains that are related in both sequence and structure can be found in several diverse protein families. Ig-like domains are involved in a variety of functions, including cell-cell recognition, cell-surface receptors, muscle structure and the immune system [<cite idref="PUB00027656"/>]. </p><p>Major Histocompatibility Complex (MHC) glycoproteins are heterodimeric cell surface receptors that function to present antigen peptide fragments to T cells responsible for cell-mediated immune responses. MHC molecules can be subdivided into two groups on the basis of structure and function: class I molecules present intracellular antigen peptide fragments (~10 amino acids) on the surface of the host cells to cytotoxic T cells; class II molecules present exogenously derived antigenic peptides (~15 amino acids) to helper T cells. MHC class I and II molecules are assembled and loaded with their peptide ligands via different mechanisms. However, both present peptide fragments rather than entire proteins to T cells, and are required to mount an immune response.</p><p>Some of the proteins in this group are responsible for the molecular basis of the blood group antigens, surface markers on the outside of the red blood cell membrane. Most of these markers are proteins, but some are carbohydrates attached to lipids or proteins [Reid M.E., Lomas-Francis C. The Blood Group Antigen FactsBook Academic Press, London / San Diego, (1997)]. Lutheran blood group glycoprotein (B-CAM cell surface glycoprotein) (Auberger B antigen) (F8/G253 antigen) belongs to the Lutheran blood group system and is associated with Lu(a/b), Au(a/b), LU6 to LU20 antigens. </p><p>More information about these proteins can be found at Protein of the Month: MHC [<cite idref="PUB00035862"/>].</p>