The correct answer is antibodies. When a virus invades the body, some anti-B antibodies cells recognize it as non-self and produce plasma cells that release a substance to kill the virus. That is why we have blood types figured out because it is important to know before any blood transfusion or transplantation, the antibodies of another person must also match with your, otherwise it's just going to be rejected by your body that might lead into some serious cases.
