Genetic basis of Blood Groups - Cats have a well-characterized AB blood group system, which includes three main blood types: Type A, Type B, and Type AB. The blood type is determined by the presence or absence of specific antigens (complex sugars) on the red blood cell surface, controlled primarily by the CMAH gene responsible for modifying cell surface sialic acids. The A allele is dominant over the B allele, so cats with one or two copies of the A allele exhibit Type A blood. Cats homozygous for the B allele have Type B blood. The rare Type AB blood group results from the presence of both A and B antigens, inherited independently and considered codominant or inherited in a complex pattern. Different mutations affecting the CMAH gene produce the blood types seen in different breeds and populations. Due to natural antibodies, Type B cats have strong anti-A antibodies, and Type A cats may have weak anti-B antibodies.
Pathophysiology - Blood groups in cats are defined by the erythrocyte surface antigens resulting from CMAH enzyme activity. Type A red cells broadly express the “A” antigenic form of neuraminic acid, while Type B cells express a distinct form “B” antigen. Type AB cats express both. Naturally occurring alloantibodies exist against the opposite blood type antigens: Type B cats have strong anti-A antibodies, and Type A cats have weak anti-B antibodies. This creates a risk of hemolytic transfusion reactions if mismatched blood transfusions occur. Incompatible matings (e.g., Type B mother with Type A father) can cause neonatal isoerythrolysis in kittens via maternal antibody attack on red blood cells ingested through colostrum.
Complications - Hemolytic transfusion reactions: If a cat receives mismatched blood, the alloantibodies attack transfused red cells, leading to acute hemolysis, shock, and possibly death. Neonatal isoerythrolysis: occurs when Type B queens produce anti-A antibodies transmitted via colostrum, which attack red blood cells in Type A or AB kittens, causing anemia and death. Blood incompatibility prevents existence of universal donors. Crossmatching and blood typing are essential before transfusions and for breeding planning.
Why This Matters to Breeders and Vets - Breeders: Blood typing of breeding cats is important to avoid incompatible matings that could lead to neonatal isoerythrolysis in kittens, a serious and potentially fatal condition. Knowing the blood type can help plan pairings safely. Veterinarians: Blood typing and crossmatching are critical before any blood transfusion in cats to prevent life-threatening transfusion reactions. Vets must be aware that most cats in many countries are Type A (~99% in the USA), but Type B is more common in some breeds and geographical regions. Type AB is rare globally. Knowledge of blood types aids in transfusion safety, breeding decisions, and management of neonatal isoerythrolysis cases.
Summary - The feline AB blood group system consists of three blood types: A, B, and AB. This system is genetically controlled primarily by the CMAH gene, with the A allele dominant over the B allele and AB showing complex inheritance. Because cats naturally carry alloantibodies against the antigens they lack, incompatible blood transfusions or blood type mismatches between queens and kittens can cause severe hemolytic reactions and neonatal isoerythrolysis. Blood typing and crossmatching are essential clinical tools to ensure safe transfusions and breeding. Breed and geographic variations influence the prevalence of blood types, influencing veterinary and breeder strategies worldwide.