Blue = Clinical Significance
Red = MCAT Tips

Blood antigens were always kind of confusing for me and it felt like everyone around me knew ALL about them. The blood antigens are important for the MCAT because if you think about it, blood in your body plays a very important role and without it we would not be able to live. For example, if someone got into a fight with someone else and then one person ended up shooting the other person, the person that was shot would have a lot of blood coming out of their wound. When this person arrives in the emergency department, the first thing that we would worry about is making sure the patient is breathing and get their airway stabilized. But the next thing that would happen is blood pressure would be checked which would most likely be low because blood is pouring out of the wound. We could give the patient some fluids and then a possible blood transfusion. Now we would not be able to just pick up any blood and give it to the patient because we have to worry about different blood types. This is why MCAT test makers like to test you on blood antigens, because it is actually applicable to real life.

So let’s take a person who is blood type A. What does that even mean? People with blood type A have a little “A” antigen on their red blood cells. Let’s back up, what is an antigen? So an antigen is a structural substance that alerts your immune system and tells the immune system to make any antibodies against it. If there is a foreign antigen (like from bacteria) in your body, your body will recognize that it is foreign and produce antibodies against it. The antibodies will then “attack” the antigen. (This would be a good point to review what antigens and antibodies are if you can not remember.) But because the immune system recognizes that the A antigen is part of the “self” the immune system does not make antibodies against itself. (There are certain diseases where the immune system can no longer tell the difference between what is self and what is foreign and that is what we call an autoimmune disease). A person with blood type A would have A antigens on the red blood cell and then because the immune system knows that the A antigens are not “bad” like a bacteria or something, the immune system would not make any antibodies against the A antigen. But here’s the thing, the immune system recognizes that there are not any other antigens on the red blood cell and decides to make sure that if anything else enters the body that the immune system is ready to attack. What this means is, the immune system will make antibodies against B antigens so that if ever there was blood type B introduced into the body, the immune system would have the B antibodies ready to go and attack the B red blood cells.