<P> The proteins encoded by HLAs are those on the outer part of body cells that are (in effect) unique to that person . The immune system uses the HLAs to differentiate self cells and non-self cells . Any cell displaying that person's HLA type belongs to that person and, therefore, is not an invader . </P> <P> When a foreign pathogen enters the body, specific cells called antigen - presenting cells (APCs) engulf the pathogen through a process called phagocytosis . Proteins from the pathogen are digested into small pieces (peptides) and loaded onto HLA antigens (to be specific, MHC class II). They are then displayed by the antigen - presenting cells to CD4+ helper T cells, which then produce a variety of effects to eliminate the pathogen . </P> <P> Through a similar process, proteins (both native and foreign, such as the proteins of virus) produced inside most cells are displayed on HLAs (to be specific, MHC class I) on the cell surface . Infected cells can be recognized and destroyed by CD8+ T cells . </P> <P> The image off to the side shows a piece of a poisonous bacterial protein (SEI peptide) bound within the binding cleft portion of the HLA - DR1 molecule . In the illustration far below, a different view, one can see an entire DQ with a bound peptide in a similar cleft, as viewed from the side . Disease - related peptides fit into these "slots" much like a hand fits into a glove . When bound, peptides are presented to T cells . T cells require presentation via MHC molecules to recognize foreign antigens--a requirement known as MHC restriction . These cells have receptors that are similar to B cell receptors, and each cell recognizes only a few class II - peptide combinations . Once a T cell recognizes a peptide within an MHC class II molecule, it can stimulate B - cells that also recognize the same molecule in their B cell receptors . Thus, T cells help B cells make antibodies to the same foreign antigens . Each HLA can bind many peptides, and each person has 3 HLA types and can have 4 isoforms of DP, 4 isoforms of DQ and 4 Isoforms of DR (2 of DRB1, and 2 of DRB3, DRB4, or DRB5) for a total of 12 isoforms . In such heterozygotes, it is difficult for disease - related proteins to escape detection . </P>

Human hla genes have as many as 100 different alleles per gene