blodtype

Blodtype, or blood type, is a classification of blood based on the presence or absence of certain antigens on the surface of red blood cells. The most widely used system is the ABO system, which divides blood into types A, B, AB, and O. A and B antigens are sugars on the cell surface; people with type A have A antigens and anti-B antibodies, type B have B antigens and anti-A antibodies, type AB has both antigens and typically no anti-A or anti-B antibodies, while type O has neither antigen and has anti-A and anti-B antibodies. The Rh system, notably the D antigen, adds a positive or negative designation: Rh-positive individuals have the D antigen, Rh-negative do not.

Genetics and inheritance: ABO blood type is inherited from two alleles (A, B, or O), with A and B co-dominant and O recessive. Expression of A or B antigens depends on the H substance. A rare Bombay phenotype lacks H antigen and can resemble type O but carries antibodies against H. RhD inheritance follows a dominant pattern, determining Rh positivity or negativity.

Medical significance: Blood type is essential for transfusions and organ transplantation. Mismatched blood can trigger transfusion reactions. In pregnancy, anti-D antibodies in an Rh-negative mother can threaten an Rh-positive fetus, a risk managed in many cases with Rho(D) immune globulin. Blood typing is performed by serology and increasingly by genetic testing.

Population variation and practical use: Frequency of blood types varies by ethnicity and region. While blood typing informs clinical decisions, crossmatching and compatibility testing are required to ensure safe transfusions.