beta2-microglobulin (beta2M) is a 12 kD nonpolymorphic Ig like protein. It is a non-membrane-anchored glycoprotein and is noncovalently associated with 39-44 kD polymorphic heavy chains of MHC class I molecules to form HLA class I antigen complex. In association with HLA class I, beta2M is expressed on all leukocytes, platelets, endothelial cells and epithelial cells. beta2M plays an essential role both in governing MHC class I molecules' stability and in promoting antigen binding and presenting the antigen to CD3/TCR complex of CD8+ T cells.