Our blood, as humans, has different varieties and features. Red blood cells transport oxygen throughout the body, while white blood cells, also known as immune cells, fight against infection.
Red blood cells have a certain kind of complex protein on the outside of the cell membrane. These proteins, known as Antigens, act as a QR code, allowing the immune system to recognize your body’s cells, without attacking them as foreign agents. These antigens come in two varieties, A and B, and they determine your blood type.

So how do we get 4 blood types from only two antigens?

The antigens are coded for by 3 different alleles, varieties of a particular gene, while the A and B alleles code for A and B antigens, the O allele codes for neither, and because we inherit one copy of each gene from each parent, every individual has two alleles determining blood type. When these happen to be different, one overrides the other, depending on their relative dominance. The A and B alleles are both dominant, while O is recessive, so A and A gives you Type A blood, while B and B gives you Type B. If you inherit one of each, resulting code dominance will produce both A and B antigens which is Type AB.

The O allele is recessive, so either of the others will override it when they are paired, resulting in either Type A, or Type B. But if you happen to inherit two O’s, instructions will be expressed that make blood cells without the A or the B antigen.
These interactions help us predict the relative probability of children’s blood type.

Why do blood types matter?
For blood transfusion, finding the correct one is a matter of life and death. If someone with Type A blood is given Type B blood or vice versa, their antibodies will reject the foreign antigens and attack them, potentially causing the transfused blood to clot. But because people with type AB blood produce both A and B antigens, they don’t make antibodies against them, so they will recognize either as safe, making them universal recipients.
On the other hand, people with blood type O don not produce either antigen, which makes them universal donors, but will cause their immune system to make antibodies that reject any other blood type.

Unfortunately, matching donors and recipients is a bit more complicated, due to additional antigen systems; particularly RH factor, named after the Rhesus monkeys in which it was first isolated.
RH+ or RH- refers to the presence or absence of the D antigen of the RH blood group system, and in addition to impeding some blood transfusions; it can cause severe complications in pregnancy. If an RH- mother is carrying an RH+ child, her body will produce RH antibodies that may cross the placenta and attack the foetus, a condition known as Hemolytic disease of the newborn.