Seven of them, A, C, D, K, P, Q, and U, are internationally recognized, with an eighth, T, which is primarily used in research.[9] Each blood group has at least two allelic factors (for example, the A blood group has a, b, c, d, e, f, and g), which can be combined in all combinations (Aa, Afg, Abedg, etc.), to make many different alleles. This means that horses can have around 400,000 allelic combinations, allowing blood testing to be used as an accurate method of identifying a horse or determining parentage. Unlike humans, horses do not naturally produce antibodies against red blood cell antigens that they do not possess; this only occurs if they are somehow exposed to a different blood type, such as through blood transfusion of transplacental hemorrhage during parturition.[10]