In immunohematology, we deal with two major entities, antigens and antibodies. Today, more than 700 blood group antigens are known to exist.
| Definition | An antigen (immunogen) is any substance that can induce a specific immunological response, i.e., cause the production of antibody. |
| Location | Blood group antigens are located in the red cell membrane. Some antigens (such as ABH) protrude from the membrane, and some (such as Rh) are embedded in the membrane (Figure 2-1). |
Each red cell has > 1 million antigens. Some antigens are evenly distributed over the red cell surface, and some are distributed in clusters. Examples include:
| A1 antigen: | 1 million antigens/rbc |
| I antigen: | 500,000 antigens/rbc |
| D antigen: | 10,000 - 30,000 antigens/rbc |
| K antigen: | 3,000 - 6,000 antigens/rbc |
In general, antigens are composed of carbohydrates, lipids, and proteins, usually a combination of any two. For example, ABH antigens are glycosphingolipids (sugars attached to a lipid backbone) and glycoproteins (sugars attached to a protein backbone). Rh antigens are lipoproteins. The chemistry of many antigens is unknown.
The biological role of blood group antigens is unknown, but one is presumed to exist. In a general sense, antigens obviously provide a way to distinguish between self and non-self, and thus play a role in immunity. But this does not explain why so many blood group antigens exist. Some antigens are associated with susceptibility or protection against diseases, but these associations are statistical and the causes are not well understood. For example, group A people have a higher incidence of cancer of the stomach, and group O people have a higher incidence of gastric/duodenal ulcers. On a trivial level, studies have shown that mosquitoes prefer group O blood, and group O people have a higher IQ than group A people, at least in the north of England. One association of importance is that the Duffy phenotype Fy(a-b-), prevalent among West Africans and American blacks (68%), acts to protect against infection by certain malarial parasites.
Terminology|
| Red Cell Antigens |
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