What regions do we test and match for in HLA?

The regions of HLA proteins that interact with T-cell receptors polypeptides (proteins) are extremely variable from individual to individual (polymorphic), and they are called Antigen Recognition Sites (ARS). The polymorphism is conferred by DNA sequences that encode those polypeptides (proteins). In Class I molecules (HLA-A, -B, -C) exon 2 and 3 of Class I genes encodes for those polypeptides and they are called ARS exons. In Class II molecules (HLA-DRB1, -DQB1 and DPB1) exon 2 of class II genes encodes the ARS regions.

ARS T Cell receptor interaction is shown below:

Therefore in HLA testing we primarily sequence those exons and predict the protein sequences (1x High Resolution). DNA sequences that result in identical protein sequences for ARS regions are considered a match.