Human leukocyte antigen B27
Homo sapiens (human)
The blood is a major transporter of cells through the body. Red blood cells transport oxygen and are responsible for the red color of blood. White blood cells, less in number than red blood cells, have a more complicated structure, including a cell nucleus and contribute to help the body fight infections. There are several types of white blood cells, one type being lymphocytes. Lymphocytes come as B cells that produce the antibodies of the immune system, or T cells. There are several types of T cells providing different functions. Cytotoxic T cells (CTL) are responsible for the destruction of infected or otherwise sick cells. The surface of T cells is covered with thousands of identical receptor molecules. These are proteins embedded in the T cell membrane. In order to recognize the cells that should be destroyed, the T cell receptors interact with molecules present on the surface of the target cell. The molecule which is recognized is called a histocompatibility molecule or human leukocyte antigen (HLA). HLA are expressed by a cluster of genes that is called the major histocompatibility complex (MHC).
HLA molecules as shown here contain two subunits. One of these two subunits is of particular interest. It is built from an 8-stranded beta-sheet forming a layer on whose top we see two parallel helices. These two helices form a long groove and inside that groove is a fragment of a protein molecule, a peptide. The peptide is the antigen that is to be recognized by the T cell. The overall shape resembles almost a plate on which the peptide is presented. Hence this complex is often called antigen presenting complex (APC). Through this construct the cell which carries the APC on its surface tells the T cells that it needs to be destroyed.
Protein Data Bank (PDB)
author: Arno Paehler