There are 8 main types of blood separated into 4 groups. The groups are A, B, AB, and O. They are grouped together by the presence or absence of what is known as an antigen. Antigens are substances within the blood that cause our immune systems to create antibodies. These antibodies then kill anything the immune system thinks is a threat. The specific antigens that create the different blood types are found on the surface of red blood cells and are known as type A and type B. They’re further separated by the presence of another type of antigen known as rH factor. If you have this rH antigen present, you’re considered positive, if not, you’re considered negative. Someone that has type A antigens and rH factor is considered to have type A+ blood. If you have both types of antigens and no rH factor, you have type AB- blood. If you have no A or B antigens then you are type O blood.