Cats have A, B and AB (rare) blood types. Most cats in the United States have type A blood - something like 99% of domestic shorthair cats have Type A blood. This ratio varies from country to country. Other cat breeds have a much higher incidence of Type B blood. As the popularity of purebred cats increases it is likely that the ratio of cats with Type B blood will also increase.
There is no "universal donor" blood type in cats - a very small amount of the wrong blood type can kill a cat if is is sensitized to the blood. This can be an issue in some cat breeds. In particular, British Shorthair, Cornish Rex and Devon Rex cats, where the percentage of Type B cats is much higher, perhaps as high as 50%. Other breeds with significant percentages of Type B blood include the Abyssinian, Himalayan, Japanese Bobtail, Persian, Somali and Sphinx breeds. (preceding information from Urs Giger, University of Pennsylvania, Kirk's XI).If a Type B queen (mother cat) gives birth to Type A kittens, severe reactions can occur in the kittens. Type B cats have strong antibodies against Type A blood. These antibodies are passed in the milk, which is very bad for any kittens that are Type A. Since A is the dominant blood type, this means that most of the kittens born will be Type A when a Type A father breeds with a Type B mother. Neonatal isoerythrolysis-- destruction of the kitten's red blood cells by the antibodies in the mother's milk -- is very likely to occur. In breeds in which significant percentages of Type B blood occur, it is wise to know in advance what the blood types of prospective parents are. Your vet can arrange to have blood types tested.