Main blood type is determined by one gene (I). In the human population there are 3 alleles: I^A, I^B, i and four phenotypes: A, B, AB and 0 blood type. The interaction between alleles are complete domination (A – dominant 0 – recessive,  B– dominant 0 – recessive) and codomination (AB).

We distinguish blood types by presence of proteins on the red blood cells’ surface. I^A allele expression causes presence of “A” antigen, I^B „B” antigen, i none of them.

In the plasma there are antibodies. Persons with A blood type have anti-B antibodies, with B blood type anti-A antibodies. When red blood cells don’t contain antigens (0 blood type) both anti-A and anti-B antibodies are produced. Persons with AB blood type do not produce any of these antibodies. Antibodies cause red cells’ agglutination and leads to thrombus origin what can be dangerous.