HLA-B belongs to the HLA class I heavy chain paralogues.
This class I molecule is a heterodimer consisting of a heavy chain
and a light chain (beta-2 microglobulin). The heavy chain is
anchored in the membrane. Class I molecules play a central role in
the immune system by presenting peptides derived from the
endoplasmic reticulum lumen. They are expressed in nearly all
cells. The heavy chain is approximately 45 kDa and its gene
contains 8 exons. Exon 1 encodes the leader peptide, exon 2 and 3
encode the alpha1 and alpha2 domains, which both bind the peptide,
exon 4 encodes the alpha3 domain, exon 5 encodes the transmembrane
region and exons 6 and 7 encode the cytoplasmic tail. Polymorphisms
within exon 2 and exon 3 are responsible for the peptide binding
specificity of each class one molecule. Typing for these
polymorphisms is routinely done for bone marrow and kidney
transplantation. Hundreds of HLA-B alleles have been described.
