For all practical purposes, cats are considered to have two blood groups: A and B. The third blood group, AB, is very rare in cats. As with humans, when giving a transfusion, the blood groups must be matched otherwise the cat's antibodies will destroy the alien blood cells.

Blood group incompatibility (BGI) can present a problem to breeders. One of the causes of Fading Kitten Syndrome is "neonatal isoerythrolysis". This means the kittens have a different blood type from their mother and the antibodies in her milk attack those kittens' red blood cells. The symptoms are jaundice, brown urine and rapid deterioration and death. In less severe cases, the affected kitten's tail-tip drops off. The diagnosis is confirmed by blood-typing the mother and affected kittens.

In controlled breeding programmes, BGI can be prevented by blood-typing the parents before mating them. Kittens receive one blood-type gene from each parent. Blood-type A genes are a dominant to blood-type B genes and the trait is autosomal (non-sex-linked).

A cat may be blood-type A but still carry the recessive gene for blood-type B. If two carriers of blood-type B are bred together, there is a probability of some kittens being blood type B.

* A cat homozygous (pure-breeding) for type A has 2 matching "A" genes
* A cat heterozygous for type A (carrier of type B) has one dominant "A" gene and one recessive "b" gene.
* A cat homozygous (pure-breeding) for type B has 2 matching "b" genes.

* A cat with type A phenotype may have the genotype AA (homozygous i.e. non-carrier) or the genotype Ab (heterozygous i.e. carrier).
* A cat with type B phenotype can only be genotype bb (homozygous).

The inheritance and full implications of the rare AB blood type is not yet understood. Note that "Ab" means a blood-type A cat carrying the recessive "b" gene and is different from a blood-type "AB" cat. Blood type B is shown by upper case "B". The recessive gene for that blood type is shown by lower case "b".

Problems occur when 2 carriers of type B are bred together and a type A female gives birth to type B kittens.

Mating together 2 cats with "AA" produces type A kittens.

Mating together 2 cats with "bb" produces type B kittens.

While it is easy to test for type B, it is harder to find out whether a type A cat is "AA" or "Ab" because current tests cannot show whether he is a type B carrier. The only way to find out is to test mate him to a known type B female.

If the type A cat is a carrier, then half of the kittens produced with a type B female will show signs of BGI. If he isn't a carrier, then none of the kittens will be at risk, but they will all be known carriers of type B. However, and it is a big however, due to the random way genes are passed on, it may take several matings to completely rule him out as a carrier.

When 2 carriers are mated together, on average one quarter of all kittens will be type B. Because type Ab queens have less anti-B antibodies than type AA queens, some of those kittens might survive. However, the queen will build up antibodies as a result of having type B kittens, and if she has type B kittens in the future, they are more likely to develop full-scale neonatal isoerythrolysis and die. The only way to tell whether a queen is AA or Ab is to test mate her to a type B male

.

Once again, if she carries the recessive type B gene, on average half of her kittens sired by a type B male will be type B and will show signs of BGI. If mated to a type Ab male, one quarter of kittens are likely to have BGI.

Some breeds are more likely than others to have type B. This may be because the foundation cats, purely by chance, didn't have the recessive "b" gene or it may be that the recessive "b" gene has been bred out of the gene pool (recessive genes are notoriously hard to eradicate as they can be carried undetected for many generations before meeting up with another carrier). 

The blood type is an additional consideration when outcrossing cats to broaden or reinvigorate a gene pool. Will the outcross cat introduce type B into a breed that previously had no incidence of type B? The same applies when hybridising cats (different blood type incidence in wild species is not yet known) or crossing different breeds or using random-bred cats to create new breeds. A highly desirable outcross cat may introduce the required traits, but may also introduce an unwanted blood type.

Kittens can be blood-typed at birth using a drop of blood from the umbilicus. If the kitten is the same blood type as the mother, it is safe for it to suckle from her. If the blood types differ, it is advisable that the kitten does not suckle from her for its first 16 hours of life as that is when its gut is permeable enough to allow antibodies from her colostrum to get into the bloodstream. Since maternal antibodies are important during the kitten's early life, hyperimmunoglobulin sera is available for kittens that are hand-reared due to Blood Group Incompatibility. After 16 hours it is considered safe to allow it to suckle from their mother.

Because there is no type O in cats, there is no universal donor. If cats receive the wrong blood type even once, a Haemolytic Transfusion Reaction occurs and can result in death. For this reason, it is sensible to know a cat's blood type, especially if it is a breed with a high incidence of type B. Usually a cat will only need a transfusion due to an emergency (blood disorder or traumatic accident) and any delay while ascertaining its blood type could be life threatening. Giving the wrong blood type is equally life-threatening.

In recipient cats given correctly matched blood, the transfused (donor) red cells survive for about 35 days. Incompatible blood cells are destroyed within 7 days.

Type B cats can die if they receive a blood transfusion containing the more common type A blood. Type B cats have medium to high levels of anti-A alloantibodies which causes the rapid destruction of transfused type A red cells. This can occur within hours and there is a high risk of anaphylactic shock.

Type AB cats are rare. Ideally, they should receive blood from a type A donor. They can receive type B blood, but as type AB cats potentially have Anti-B alloantibodies, this may result in premature destruction of the type B red cells in donated blood.