Which statement best defines histocompatibility leukocyte antigens (HLA)?

• A. Cell surface proteins that identify self from nonself
• B. Chemical mediators that induce a particular response from lymphocytes
• C. Tumor markers that identify cells as malignant
• D. Surface markers that stimulate differentiation and proliferation of different blood cell lines in the bone marrow

Answer: (A)

Histocompatibility leukocyte antigens are cell surface proteins encoded by HLA genes that form the major histocompatibility complex. They are expressed on nearly all nucleated cells and present peptide fragments to T cells, allowing the immune system to distinguish self from nonself. This self/nonself recognition is essential for tissue compatibility in transplantation—the closer the HLA match between donor and recipient, the lower the risk of graft rejection. The other statements describe different immune components (cytokines as mediators, tumor markers, or growth/differentiation signals) and do not define what HLA are.