T-Cell Receptor and Fab Fragment

Immunoglobulin domains are not only components of the Immunoglobulins, but also appear as subunits of the T-cell receptor extracellular domain, which looks like a membrane-linked Fab fragment, as well as in its binding partner, the HLA or MHC complex and in many other molecules involved in the regulation and maturation of cells of the immune system, eg. CD2, CD4, CD8.