ABO incompatibility is a reaction of immune system that takes place when two types of blood that are incompatible are mixed up. As we all know, the three major blood types are A, B, and O. The types are determined on the basis of small substances called molecules on the surface of the blood cells. These molecules of people who have different blood types play the role of immune system triggers in the body.

Each person of Type A, Type B and Type AB has a combination of two of these surface molecules. Type A is made up of AA or AO molecules. Type B is made up of BB or BO molecules. Type AB is made up of one A and one B molecule. Yet, Type O doesn’t have any molecule. The symptoms of ABO incompatible transfusion reactions include back pain, blood in urine, feeling of "impending doom", fever as well as yellow skin.

Typically, the antibodies which act against the foreign blood types A and B and flow in mother's circulation are still there, as they are too big to go through the placenta easily into the fetal blood flow. A few fetal red cells usually go into mother's blood circulation through the placental barrier. Although mother’s blood is incompatible with fetal blood theoretically, it is really a big truth they can be mixed together to some extent.

It may result in a reaction to mix two different types of blood together. It is really a important point for a patient who is going to receive blood or have an organ transplant. The blood types should be tested to tell whether they are incompatible to get rid of an ABO incompatibility reaction.

As there aren’t any surfaces molecules on the surface of type O, type O blood will not result in an immune response on the basis of ABO incompatibility. Therefore, patients of any blood type can have blood of type O. People who have blood of type O are recognized as universal donors. Yet, they just can receive the blood of type O.

Two blood types that are incompatible with each other cause complications when being mixed up. The reason why blood types are different from each other lies in small molecules on the surface of the blood cells. There is a combination of two of these molecules on the cells’ surface in every person. Unlike the other combinations of blood types, there are not any molecules on the surface of type O blood cells.

Author's Bio: