I saw that you said nowhere did the results say you were HLA-B27 positive. Instead they gave just three SNPs within the HLA-B gene without comment.
HLA-B27 Human Leukocyte Antigen B27 is located on Chromosome 6.
HLA-B27 is one of 60 or so HLA-B alleles, and is a Major Histocompatibility Complex (MHC) Class I gene that encodes a peptide binding protein (in this case HLA-B27) that are expressed on all nucleated cells. The molecules consist of an Î± chain and Î²2microglobulin (not part of MHC) and present ligands (antigen fragments of 8-10 amino acids long, usually virally derived or â€˜selfâ€™, but also can be derived from intracellular bacteria).
HLA-B27, like the all the HLA-B so called 'antigens' were originally defined serologically. That is, the serum of multi-parous women (multiple births) were run against panels of white cells from different people and eventually antiserum generated that could react with different groups of white cells so that the cells were designated as displaying surface markers HLA-B1 through to B60 (some are now redundant). We each have two HLA-B genes (one from each parent) so I might be HLA-B27/B14 for instance.
Since those days HLA typing has been refined and explained by different patterns of amino acids over stretches of hundreds of amino acids that comprise a single B type. This has led to further extensive subtyping within HLA-B27. This explains why a genome scan looking for SNPs will not be capable of readily identifying your 'B'status. Instead that is performed by a standard lab and reported as either HLA-B27 positive or HLA-B27 negative (eg. B14/B58)
The Major Histocompatibility Complex (MHC) is a large genomic region of 3.6Mb (3.6 million base pairs) situated on chromosome 6 containing 224 known genes, most having immune or related functions. The high allelic diversity in the MHC, particularly at HLA loci, is thought to provide a survival advantage due to the likelihood of heterozygosity increasing the range of antigens that can be presented.
The proteins encoded by the HLA genes are expressed on the surface of cells where they display both self antigens (short peptide fragments from the cell itself) or non self antigens (such as fragments of invading microorganisms) to a type of leukocyte or white blood cell called T-cells (lymphocytes) that have the capacity to kill or facilitate the killing of pathogens and infected or malfunctioning cells.