There exist 4 main types of blood A, B, AB, and O. What blood you possess depends on what antigen is in red cells and what antibody is in the plasma; more on that later. Blood type is hereditary and which one you possess, depends on what your parents possess. If both parents have B blood type, the child will most likely have a B blood type as well. Similarly, A and A-type parents will produce offspring with A-type blood. However, if one parent is A-type and the other B-type, the child will be AB-type. Everything gets trickier when we introduce the O blood type. A and B blood types are dominant genes, or “alleles” to be more exact. Whereas the allele responsible for O blood type is recessive. Which means if one parent has an O blood type and the other parent has either an A or B blood type, then the child is most likely to possess an A or B blood type. In order for someone to acquire an O blood type, both their parents must have O blood types as well. Making O blood types notably limited because O blood type is recessive unlike its A and B counterparts. Now back to what I was discussing previously, for example, A blood type will have A antigens on the blood cell and have B antibody in the plasma, same goes with B blood type. However; AB blood types will have neither A nor B antigens on red cells and both A and B antibody are in the plasma. Finally, O blood type has neither A nor B antigens on red cells however both A and B antibody are in the plasma. So up to now, we have talked about four blood types, A, B, AB and O. However, there are actually eight blood types, at least within the ABO system. Those are A+, A-, B+, B-, AB+, AB-, O+ and O-. So where do the positives and negatives come from? It comes from the third type of antigen, called the Rh factor. Your blood cells either have the Rh factor antigens or they don’t. If the Rh factor antigens are present, then you have a positive blood type, if they’re not present you have a negative blood type. Rh negative blood may be given to patients with either Rh negative or Rh positive blood types, providing their primary A, B or O blood type matches, as we talked about previously. However, Rh positive blood will only be accepted by someone who also has Rh-positive blood.