Cats have one blood group system with three blood types: type A, type B, and type AB. Based on the fact that each individual has two sets of chromosomes, these are due to different forms (alleles) of the same gene. Thus, only cats carrying two copies of the B allele (genotype homozygous B/B) will have blood type B. Cats with blood type A may have two copies of the A allele (homozygous AA) or one only copy (heterozygous A/B). Type A is completely dominant over type B. The third blood type, type AB, appears to be a third form of the same gene, but it is rare. Type A is the most common feline blood type, present in up to 94%-99% of all domestic shorthair and longhair cats in the United States.
The frequency of the feline blood groups varies both by breed and by location within the United States. The lowest frequency of type B cats is in the Northeast and North Central/Rocky Mountain regions. Higher frequencies of type B cats are found on the West Coast, peaking in the Northwest with 6% type B cats.
Siamese cats and related breeds with oriental blood have thus far all been shown to have type A blood. The American Shorthair breed, due to its close relationship to non-pedigreed shorthair cats, is also largely blood type A. However, some other breeds may have astoundingly high numbers of type B cats. The frequency of the blood types does not vary geographically for pedigreed cats.
All blood type B cats have strong antibodies against type A blood cells as of three months of age. Blood type A cats generally have very low anti-B antibody titers. It is very important to note that these antibodies are naturally occurring; unlike other species, no previous pregnancy or transfusion is necessary for antibody development. The strong anti-A antibodies in type B cats are important in two situations: blood transfusion reactions following the administration of type A blood to type B cats and neonatal isoerythrolysis (NI or hemolysis of the newborn) due to the newborns’ type A or AB erythrocytes being attacked by the anti-A antibodies in the type B queen’s colostrum. The anti-B antibodies have not been shown to cause NI, but can lead to transfusion reactions if type B blood is given to type A cats. Fortunately these situations can now be avoided. [Read complete article]