| The human leukocyte antigen system (HLA) is the major histocompatibility complex (MHC) antigen
system in humans. "HLA" is used in conjunction with a letter and number (e.g. HLA-B27) to designate a specific allele at a given
HLA locus.
The proteins on the outer part of body cells that are (effectively) unique to that
person. Any cell displaying that person's HLA type belongs to that person (i.e., is not an invader). Any cell displaying some
other HLA type is "not-self" and is an invader. HLA types are inherited, and some of them are connected with autoimmune disorders and other diseases. Scientists think that people
with certain types of HLA antigens are more likely to develop certain autoimmune diseases, such as SLE (Lupus), Myasthenia
Gravis, and Sjogren's Syndrome.
When a foreign pathogen enters the body, specific cells called antigen presenting
cells (APCs) grab that pathogen inside. Proteins from the that pathogen are chewed up into small pieces (peptides) and loaded onto HLA antigens (further classified into class I MHC and class II MHC antigens). They are then displayed to the cells of the immune system (T cells) which then produce a variety of effects to eliminate the pathogen. Pathogens acquired from outside
the cells (such as bacteria) are processed on MHC class II HLAs whereas pathogens
made inside the cells (such as viruses) are processed onto MHC class I HLAs. This
distinction is very important for body's immune system to know because it helps immune system make directed and effective immune
response.
Besides working HLA antigens, in humans there are two additional HLA antigens, HLA-DM and HLA-DO, which are important in
loading the antigenic peptides generated from pathogens onto HLA molecules on antigen presenting cell's cell surface.
HLA types were key to matching tissues and organs for donation and organ transplantation under outdated immunosuppression protocols; currently only zero-mismatch (six
haplotype match) is generally regarded as relevant. Direct crossmatch and blood type matching are still used.
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