An "HLA Typing of HLA-A, HLA-B, and HLA-DRB1" test is a genetic analysis that determines the specific variations (alleles) of the HLA-A, HLA-B, and HLA-DRB1 genes on a person's DNA, which are crucial markers on the surface of cells that the immune system uses to identify "self" and "foreign" cells, making this test vital for organ transplantation compatibility checks to minimize rejection risk.