The 4C9 monoclonal antibody reacts with rat beta 2 microglobulin (β2M) a 12 kDa component of MHC class I molecules. β-2-microglobulin, a non-membrane-anchored glycoprotein, noncovalently associates with the polymorphic heavy chain of MHC class I molecules to form the HLA class I antigen complex. β-2-microglobulin is expressed on all leukocytes, platelets, endothelial cells and epithelial cells and plays an essential role in mediating proper folding and expression of MHC class I molecules. The importance of β-2-microglobulin is illustrated by β-2-microglobulin knockout mice which show a normal distribution of T lymphocytes, but have no mature CD4 or CD8 T cells and are therefore defective in CD4 and CD8 T cell-mediated cytotoxicity. The 4C9 antibody recognizes rat β-2-microglobulin alone or complexed with FcRn heavy chains.