The nomenclature of HLA typing is complicated. HLA can be classified by serology at the antigen level and/or at the DNA level by nucleotide sequencing.
As antigens were detected by serology, HLA were named in the numerical order in which they were discovered. HLA-A1, A2, A3, A4, etc. However, scientists and researchers realized that more than one HLA locus exists. Class I HLA are: HLA-A, B, C. Class II HLA are loci: DPB1, DQB1, DRB1, and DRB3, DRB4, and DRB5.
HLA expression is not uniform across all cells. Almost all nucleated cells possess class I HLA. HLA class II is only present on antigen-presenting cells.
