Blood group antigens are carbohydrates that are attached to proteins or lipids. An antigen is a substance foreign to the body that causes an immune response. An immune response occurs when antibodies, which are proteins in your immune system, are summoned to attack an antigen.
When you say you are blood type A, what you are telling people is that the cells in your body make antibodies only to type B antigens. The A-type surface antigens on the cells are not recognized. These surface antigens can be attached to the surface of your blood cells (more specifically to the plasma membrane surrounding the cells) or to proteins or lipids anywhere in your body.
That means that your body makes antibodies against type B antigens. (If your blood type is positive or negative, that refers to the Rh factor.) So, in essence, your body kills off the cells containing type B antigens, allowing type A to be dominant. You can receive type A blood or type O blood and can donate blood to those with type A or type AB.
If you are blood type B, the situation is reversed. Your cells have type B antigens attached, so your body makes antibodies against only type A. Once the type A antigens are kept at bay, your blood cells “show” type B as the dominant type. You can receive type B or type O blood, and you can donate to those with type B or type AB blood.
If you are blood type AB, your cells do not make antibodies against type A or type B surface antigens. Therefore, you can receive blood from a donor with any blood type (universal recipient), but you can donate blood only to other people with type AB blood.
If you are blood type O, your cells make antibodies against both type A and type B antigens. This means that if you need blood, you can only receive more type O blood. But, you can donate your blood to anybody; thus, you are a universal donor. Type O blood is the most common.
