Blood groups describe the various combinations of antigens present on our red blood cells—erythrocytes. These antigens are located on the outer surface of the erythrocyte membrane. They are genetic traits inherited from parents and remain unchanged throughout a person's life.
According to the ABO system introduced by Austrian physician Karl Landsteiner, there are four blood groups. This system is based on the identification of two antigens—A and B. If neither antigen is present, the blood group is 0, or the first group. When antigen A is present, it is referred to as the second blood group, or group A. With antigen B, an individual has the third group, group B. The combination of both antigens results in group AB, or the fourth group.
However, even within these primary blood groups, there are subtle differences arising from mutations in the responsible genes. As a result, the second and fourth groups have been subdivided into subgroups. It has been discovered that individuals from one of these subgroups are at an increased risk of cardiac events.
A study published in the journal Neurology found a clear link between the A1 gene (a subgroup of the second blood group) and early strokes.
The authors gathered data from 48 genetic studies involving around 17,000 stroke patients and approximately 600,000 control subjects without strokes, aged between 18 and 59 years. The researchers then reviewed all collected chromosomes to identify genetic variants associated with strokes.
After adjusting for sex and other factors, experts found that individuals with the second blood group had a 16% higher risk of stroke by age 60. Conversely, those with the first group had a 12% lower likelihood of stroke compared to individuals with other blood types.
Researchers are currently unable to precisely explain this pattern. However, they suggest it may be related to blood clotting factors that play a role in thrombus formation.
For instance, a study published in the Journal of Biosciences and Medicines indicated that clotting factor VIII (known as anti-hemophilic factor) was significantly higher in the second, third, and fourth groups compared to the first. However, no significant differences were found among the three groups regarding clotting factors.
Additionally, scientists discovered that late strokes occur less frequently in individuals with the second blood group. This may be attributed to the fact that early strokes have a different mechanism. Researchers state that in young people, vascular events are most often caused by thrombus formation and much less frequently by atherosclerosis, which involves the accumulation of fatty deposits in the arteries.
One study showed that subgroup A1 and group B are closely linked to the risk of venous thrombosis, while subgroup A2 is associated with a moderate increase in risk compared to subgroup 01.