Different blood types are determined by the different surface antigens that surround red blood cells. These antigens are combinations of sugars and proteins that remain on the surface of red blood cells (RBCs). In the ABO blood typing system there are only two surface antigens that can be attached to RBCs, type A antigens and type B antigens. People with blood type O have neither type A or B antigens on the outside of their RBCs, while people that have blood type AB have both type A and B antigens on their RBC surface. These surface antigens are actually spread throughout the body, not just on red blood cells and they work in a fundamentally different way than most other antigens.
Our bodies usually make antibodies only when we have been exposed to an antigen. With almost all other antibodies found in the body we have to have an initial contact with the antigen before our body creates the corresponding antibody. However, with ABO blood types we are born with antibodies for antigens that we do not have in our body. So someone with type A blood would have anti-B antibodies which would react if they were given type B blood and vice versa for someone with type B blood. The reason for the early presence of these antibodies in our blood is not fully understood and theories for their presence abound. One of the more credible theories is that we are exposed to the antigens in our environment without us knowing it at some very early part of our upbringing.
Since your blood type is inherited, you get one gene for your father’s blood type and one gene for your mother’s blood type to combine and form your blood type. So someone with type A blood would have to have a mother or a father that had type A blood also. If you are type A then you would have you would be genetically coded for either two type A genes, or a type A and a type O gene. The same setup works with type B. Someone that is blood type AB would have both a type A gene and a type B gene. People with blood type O would have two genes that don’t code for either type A or type B antigens. What that means for people with type O blood is that they carry both anti-A and anti-B antibodies, which means that they cannot receive transfused blood from someone with either type A or B blood. Blood types are also referred to as being “positive” or “negative” which indicates another type of blood antigen called Rhesus factor. An article on Rhesus factor can be found here.