The antibody recognizes the hidden determinant of β-2 microglobulin (i.e. binding to its determinant is available only when the chain is separated from the HLA heavy chain. β-2 microglobulin is a 12KDa protein with a pI of 5.6. Serum β2 microglobulin levels are a reflection of cell turnover. Levels rise with fever, inflammation, and infection. Increased serum levels are also seen in B-cell malignancies and in renal failure and may indicate a worse prognosis for patients with early-stage Hodgkin’s lymphoma. In urine, increased levels are seen in proximal renal tubular disease as well as renal transplant rejection. β2 microglobulin levels can rise either because its rate of synthesis has increased (e.g. in AIDS, malignant monoclonal plasma cell dyscrasia, solid tumours and autoimmune disease) or because of impaired renal filtration (e.g. due to renal insufficiency, graft rejection or nephrotoxicity induced by post-transplantation immunosuppressive therapy).