Genes encoding proteins containing one or more immunoglobulin (Ig) domains (homology units) that are homologous to either Ig variable (V) region or constant (C) region domains. Cell surface and soluble molecules mediating recognition, adhesion, or binding functions in and outside the immune system, derived from the same precursor, belong to this family of molecules (e.g., immunoglobulin, T-cell receptor (TCR), major histocompatibility complex (MHC) class I and class II molecules, CD4, CD8, and $\ce{Fc\upgamma\!R}$).