Human blood types differ due to the presence or absence of specific molecules called antigens on the surface of red blood cells.

These antigens are inherited traits encoded by genes, defining major blood groups such as ABO and Rh.

Genetic Basis of Blood Types

Blood type classification primarily depends on antigens, which are complex molecules made of proteins and sugars present on red blood cells. The ABO blood group system divides human blood into four main types: A, B, AB, and O, depending on which antigens are expressed. For example, type A has the A antigen, type B has the B antigen, type AB has both, and type O has neither.

These antigens are produced by enzymes coded by different alleles inherited from parents, making blood type a hereditary trait.

In addition to ABO, the Rh factor, another important blood marker, indicates the presence (Rh positive) or absence (Rh negative) of a different antigen on red blood cells. The combination of ABO and Rh status creates a spectrum of possible blood types, influencing compatibility for blood transfusions and donation.

The molecular and genetic mechanisms that determine these antigens are highly conserved and have evolved over millions of years, ensuring the integrity and functionality of red blood cells.

Evolutionary Drivers of Blood Type Diversity

The diversity of blood types is not random but has been shaped significantly by evolutionary pressures, primarily related to infectious diseases. One of the most compelling evolutionary explanations involves malaria, a deadly parasitic disease caused by Plasmodium species.

Research shows that individuals with type O blood are less susceptible to severe malaria, providing a survival advantage in malaria-endemic regions. Consequently, type O blood is more prevalent in populations from areas historically affected by malaria

Other infectious diseases have also influenced blood type frequencies. For instance, certain blood types correlate with varying susceptibility or resistance to bacterial or viral infections. Blood group A has been linked to higher vulnerability to smallpox, while type B is associated with infections like gonorrhea and tuberculosis.

This suggests that different blood types offered distinct immunological benefits or risks, contributing to their maintenance in human populations through natural selection.

The Role of Antigens Beyond Blood Compatibility

Blood group antigens are more than identifiers for transfusion safety; they play active roles in immune response and cell recognition. The sugar molecules on red blood cells interact with pathogens and immune cells, affecting infection dynamics. Some pathogens exploit these antigens to attach and invade cells, while others are blocked by the particular antigenic profile of the host's blood.

Moreover, research suggests that blood types may affect other physiological processes and disease risks unrelated to infections. For example, some studies associate blood groups with variations in clotting tendencies, cardiovascular health, and susceptibility to conditions like diabetes. While these associations are complex and not fully understood, they underscore the multifaceted nature of blood type biology.

Dr. Claudia S. Cohn, MD, PhD, is a distinguished physician specializing in transfusion medicine, states "The data are strongly suggestive that the whole reason we have different blood groups is malaria."

Humans have different blood types because of inherited genetic variations that determine specific antigens on red blood cells. These variations are a product of evolutionary forces, mainly driven by the need to survive diverse infectious threats such as malaria. Understanding the origins and functions of blood types reveals the intricate interplay between genetics, environment, and human health over millennia.