Until the 20th century, it was widely believed that all human blood was identical. However, this notion was dispelled when it was revealed that there are distinct blood types: A, B, AB, and O, each with positive and negative variants. The divergence in blood types is intricately connected to our immune system.
As you may be aware, our blood harbors white blood cells that furnish immunity against external threats such as bacteria and viruses. The term "type" denotes the presence of a specific antigen protruding from the surface of a red blood cell. An antigen is any substance that triggers a response from an immune cell called an antibody. Antibodies attach themselves to foreign agents, such as bacteria and viruses, and aggregate them for elimination by other components of the immune system. The human body naturally produces antibodies designed to target specific types of red blood cell antigens.
To illustrate, if your blood group exclusively contains type 'A' antigens, it is categorized as type A. If both A and B antigens are present, it is designated as type 'AB.' Conversely, if no antigens are present, it is classified as 'O' type. Additionally, the presence or absence of another antigen called the Rh factor is denoted by the + or - sign alongside your blood type. When blood with different antigens is introduced into an individual, their immune system identifies it as an 'enemy' and mounts an attack. This elucidates why O negative is considered the universal blood type.
The subsequent inquiry delves into the evolutionary aspect. Indeed, blood types are believed to have evolved approximately 20 million years ago, although the precise factors driving this evolution remain under research scrutiny. Finally, if everyone shared the same blood group, it would imply a uniform set of antibodies among all individuals. Consequently, there would be no need for blood type testing prior to transfusions. In essence, a straightforward concept, wouldn't you agree?