This MAb recognizes the HLA-B27 cell surface antigen on human cells. It may be used to HLA type human lymphocytes. Approximately 60% of patients with ankylosing spondylitis are HLA-B27 positive. This reagent can be used to help identify this HLA haplotype in human lymphocytes. Major histocompatibility complex (MHC) molecules form an integral part of the immune response system. They are cell-surface receptors that bind pep- tides and present them to T lymphocytes. Human leukocyte antigens (HLAs) are polymorphic members of the MHC family that are specifically involved in the presentation of antigens to the T cell receptor. There are two classes of HLA antigens: class I (HLA-A, HLA-B and HLA-C) and class II (HLA-D). Class I molecules are expressed in nearly all cells and play a central role in the immune system by presenting peptides derived from the endoplasmic reticulum. The differential structural properties of MHC class I and class II molecules account for their respective roles in activating different populations of T lymphocytes. HLA-B encodes a membrane anchored heavy chain, which hetero-dimerizes with a light chain (? ?-2-Microglobulin) to form MHC-I. Polymorphisms yield hundreds of HLA-B alleles. The HLA-B27 allele appears with increased frequency in uveitis patients.