Just like humans, animals also have blood types. In humans, categorization of blood types is antigen-dependent. The presence of antigens forms the basis of classification in the human blood group system. Antigens are substances that are attached to the surface of red blood cells, and stimulate an immune reaction when they combine with a different blood type. Antigens of a specific blood type produce antibodies that invade foreign antigens of an incompatible blood group. Although animal blood also contain antigens, their structure is not the same as those present in human blood. Therefore, blood group system in animals is different from the one recognized in humans. Animal Blood Types Feline Blood Types In felines, we have the AB blood group system, in which the 3 types identified are the A, B, and AB blood types. Of these 3 types, the A blood type is the most common, prevalent in almost 94% - 99% of short and long-haired cats in the United States. The B blood type is relatively common in other breeds of cats, with the AB blood type being the most rare. The A blood type contains antibodies that may harm the red blood cells in the B blood type, but they are weak as compared to the antibodies present in the B blood type, which contains antibodies that can kill the red blood cells present in the A blood type. The AB blood type is free from any antibodies. Therefore cats with AB blood type are regarded as universal receivers. This means that blood from cats having either A or B type blood can be transfused to cats having AB type blood. There have been cases where the blood type of the mother cat and its kitten are different. This can happen when a mother cat having B blood type gives birth to kittens with A or AB blood type. This inborn blood group mismatch is referred to as neonatal isoerythrolysis. When kittens born with this defect feed on their mother's milk, the antibodies present in the milk can destroy the red blood cells of the kitten. In such circumstances, the kitten can fall critically ill and death may ensue. In fact, 'neonatal isoerythrolysis' has led to a large percentage of deaths in kittens. Jaundice is one of the initial symptoms observed in kittens affected with neonatal isoerythrolysis. Canine Blood Types Canine blood types are classified into 8 major groups depending on the type of antigen (also referred to as dog erythrocyte antigen (DEA)) present in the blood. A numeric system is employed to name different antigens, which in turn helps to distinguish different blood types in dogs. They are mentioned below:

• DEA 1.1
• DEA 1.2
• DEA 3
• DEA 4
• DEA 5
• DEA 6
• DEA 7
• DEA 8

Out of these 8 canine blood types, DEA 1.1 holds a lot of significance as far as blood transfusion is concerned. Dogs with DEA 1.1 positive blood type are considered to be the most compatible as they can safely accept blood of any type for the first time, without any fear of serious hemolytic transfusion reaction. On the other hand, DEA 1.1 negative donors are regarded as universal donors, meaning they can give blood to dog with DEA 1.1 negative or positive blood type. However, these dogs are least compatible because only DEA 1.1 negative blood type can be transfused into them. It is observed that most Greyhounds, Irish Wolfhounds, Dobermans, German Shepherds, Boxers, and Pit Bulls exhibit DEA 1.1 negative blood type, indicating their universal donor status. However, in general a large percentage of dog population has DEA 1.1 positive blood type. Horse Blood Types Horse blood types, better known as equine blood groups, are classified into 8 major blood systems. Despite 30 blood types in horses, only 8 blood groups have been recognized globally, namely A, C, D, K, P, Q, U, and T. It is observed that antigens of A, C, and Q blood types are known to trigger a severe immune response when they combine with foreign red blood cell antigens. These blood groups (A, C, and Q) are considered to be the most troublesome and hence should never be donated to mismatched recipients. Also, every blood group in horses are polymorphic. For instance, saying that a specific horse has A blood type is inadequate as far as blood transfusion is concerned. This is because A blood type in horses can be present in different forms such as aA', aA'H or aH. So, in horses it is very difficult to find a 100% match between donors and recipients. Cattle Blood Types 11 important blood types have been internationally recognized among cattle. They are A, B, C, F, J, L, M, R, S, T, and Z. The B blood type can be present in several forms as there are more than 60 antigens in it. As a result, it can be quite problematic to ensure blood compatibility between donors and recipients of B blood type. The antigen in J blood type is a lipid and soluble in bodily fluids. Also, this antigen is not present in newly born offspring and is obtained when calves feed on mother's milk for the first 6 months. Sheep Blood Types In sheep, the blood group system has been classified into 7 major types, namely A, B, C, D, M, R, and X. As observed in cattle, the B blood type also exhibits several forms. Also, the antigens in R blood type of sheep are found to be soluble in body fluids. Goat Blood Types 5 important blood types have been recognized internationally in goats. They are A, B, C, M, and J. Studies show that the structure of antigens in goat blood systems is very similar to those found in sheep blood systems. Mismatched transfusion in the blood group system of animals can be life-threatening to the recipient. Hence, blood group compatibility is very important when it comes to blood transfusion in animals. Veterinarians often recommend animals that require transfusion to be blood typed. A blood crossmatch that checks the compatibility between the donor and recipient should be done to ensure safe transfusion. Disclaimer: The information provided in this article is solely for educating the reader. It is not intended to be a substitute for the advice of a veterinarian.