Blood Type A

Blood type A is one of the four types in the ABO blood group system, which is the most important method of classifying human blood. A person with type A blood has red blood cells bearing antigen A on their surface—a molecule on the cell membrane that the immune system recognizes as part of the body's own cells.

People with type A blood usually carry antibodies against antigen B. For this reason, if type B blood is transfused into a type A person, the transfused cells can clump and break down. This is why every transfusion is preceded by a test to check compatibility.

Genetic basis

The ABO type is controlled by a gene on an autosome. The A allele codes for an enzyme that synthesizes a carbohydrate specific to antigen A. Each person inherits one copy from each parent. When combined, a phenotypically type A person may carry two A alleles or one A allele and one O allele, which does not produce antigen.

Subtypes

Type A is not uniform. There are subtypes, the most common being A1 and A2, which differ in the quantity and structure of antigen on the cell. These differences can sometimes produce unclear results in serological testing and are taken into account by blood banks.

Relationship to the Rhesus system

The ABO type is usually accompanied by the Rhesus (RhD) marker, written as A+ or A−. The two systems are independent, but together they determine blood compatibility. The Rh factor is also considered in pregnancy management, because a mismatch between mother and fetus is possible.

Distribution in populations

The frequency of type A varies across regions of the world. It is generally one of the more common types in Europe and parts of Asia, while less common in other areas. Variation in type frequencies is used in studies of human history and migration.