Feline Blood Groups
There are three well-known, clinically important blood groups in cats: A, B, and AB.1-2 Despite the nomenclature, the antigens in the feline AB blood group are not serologically related to the human ABO blood
group antigens. Another potentially important group called MiK has recently been identified.3 The blood groups are genetically determined species-specific red blood cell surface antigens. The A-allele is dominant over
the b-allele so that cats with genotypes A/A and A/b will be type-A, while only the homozygous b/b will have the type-B phenotype.
A third type, AB, occurs rarely and expresses both the A and B antigens.4 However, the heritability of type-AB is not well understood.
Various methods are now available to determine blood type, both in a referral laboratory setting and patient-side. Diagnostic
laboratories use various serological methods based on agglutination reactions. In addition, genetic testing is now available
to identify blood types A and B using buccal swabs, although it cannot distinguish between A and AB blood groups.5 Patient-side testing may be performed with a card typing system (RapidVet®-H, DMS Laboratories, Flemington, NJ). If the
card-typing system is used, type-AB and type-B results should be confirmed by a referral laboratory as some cross-reactions
have been known to occur.6 A recently introduced option for patient-side blood typing is the gel column agglutination test (DiaMed-Vet® feline typing
gel, DiaMed, Switzerland). This test is easier to interpret than the card method, although it requires a specially designed
centrifuge that may be cost-prohibitive in some settings.7 An evaluation of various blood typing methods for the cat concluded that the gel column test is reliable when compared to
the gold standard Penn tube assay.6
The distribution of feline blood types varies by geographic region and breed (Table 1).8-9 Type-A is the most common type among most cats. There is, however, geographic variation in the prevalence of type-B domestic
shorthaired cats. Over 10% of the domestic shorthair cats in Australia, Italy, France and India are type-B. Breed distribution
does not vary as much by location because of the international exchange of breeding cats. Over 30% of British Shorthair cats,
Cornish and Devon Rex cats, and Turkish Angora or Vans have type-B blood. In contrast, Siamese and related breeds are almost
exclusively type-A. Ragdoll cats appear to be unique with regard to blood types. Approximately 3.2% of Ragdoll cats are discordant
for blood group when genotyping is compared to serology, necessitating further investigation in this breed.9
Table 1: Selected Blood Type A and B Frequencies in Cats (ignoring AB blood types)
The AB blood type is very rare while the frequency of the MiK blood type is unknown. The presence of red blood cell antigens
in addition to the AB group may explain why transfusion compatibility is not guaranteed by blood typing; crossmatching is
recommended prior to any transfusion.3 Breeding queens, along with blood donors and, if possible, blood recipients should be blood typed.
Understanding feline blood groups is important because, unlike other mammals, cats produce naturally occurring antibodies,
called alloantibodies, against the red blood cell antigens not present on their own cells. The kitten produces these alloantibodies
around two to three months of age resulting from the exposure to antigens on plants, bacteria or protozoa that are structurally
similar to the red cell antigens. No alloantibodies are produced against antigens that are similar to self-antigens and no
previous exposure to blood products (e.g., transfusion or pregnancy) is necessary to produce the alloantibodies. Type-A cats
may have low levels of naturally occurring antibodies against blood type-B red cells, but all type-B cats have high levels
of naturally occurring anti-A antibodies. Blood type-AB cats do not have alloantibodies.