Blood types are determined by the presence or absence of two antigens, A and B, on the surface of red blood cells, as well as the presence or absence of the Rh factor, which can be either present (+) or absent (-) . There are four main blood types: A, B, AB, and O. The compatibility of blood types for transfusion or donation depends on the presence or absence of these antigens and the Rh factor. Here is a summary of blood type compatibility:

• Type A: Can receive blood from type A and type O donors, but not from type B or AB donors.
• Type B: Can receive blood from type B and type O donors, but not from type A or AB donors.
• Type AB: Can receive blood from all blood types (A, B, AB, and O).
• Type O: Can only receive blood from type O donors, but can donate blood to all blood types (A, B, AB, and O) .

In addition, there are some programs available to help donor/recipient pairs with blood types that are otherwise incompatible, such as paired exchange and plasmapheresis. It is important to match blood types correctly for a safe transfusion or donation, as the wrong blood type can cause serious problems.