Blood groups in cats.

Susannah A-T asks "...do cats have blood types, like humans? How do veterinarians collect the blood for transfusions.?"
Interesting question, Susannah. Yes, there are blood types in cats, although the classifications and sub-types are much simpler in pets than in people. The red blood cells (RBC) themselves are essentially identical within a species; what differs is the genetically controlled antigens on the RBC membrane. This means that the blood groups are inherited from the previous generation, some dominant and some recessive. The main difference between the dominant and the recessive inheritance is that for the trait to be seen in the offspring, the recessive trait must be inherited from both parents, whereas the dominant can be from either or both parents. Now that I have confused everyone, including myself, let's go on to Susannah's question!
Cat blood groups are the simplest of all the domestic species with only two major blood groups, A and B. Cat populations tested in the United States have shown that about 99% have blood group A, so the risk of an incompatible transfusion is very low. However, some breeds of cats, such as the Abyssinian, Birman, British Shorthair, Devon Rex, Himalayan, Persian, Scottish Fold and Somali, have a higher incidence of blood group B. Incompatible transfusions leads to rapid destruction of transfused RBC, which, of course, means the transfusion will be of little help in meeting the need for RBCs in the recipient. In terms of blood to weight ratios, cats have only 4% of their body weight as blood.
A procedure known as a direct crossmatch is used to detect antibodies in the patient's blood that would cause a transfusion reaction. Such a reaction results in the breakup of the RBC cell membrane and loss of function of the cell. Blood typing may also be utilized to identify which blood group is present in an animal.
Whole blood transfusions are not used commonly in animals. Most of the time, the needs of the circulation can be met with fluids to expand the volume of blood when blood loss has occurred. The advantages of using such fluids include low cost, freedom of possible blood-borne disease transmission, ready availability and lack of transfusion reactions. Specific blood components are also sometimes used. Blood substitutes are becoming available and will probably become commonplace in the future.
Donor animals are often identified and maintained by veterinary hospitals to provide emergency transfusions to critical patients. Such animals have a happy and comfortable life, usually with free run of the hospital and naturally become staff favorites. Donations are uncommon, but in an emergency, they are invaluable. Collection of relatively small volumes of blood are accomplished using specialized blood bags, just like those used when you donate blood for human emergency needs.