Secretor

Secretor is the name given to the condition that a person secretes their blood-type antigens into saliva and other bodily fluids. It is also the name of the gene that causes this to happen.

Blood type (ABO) is determined by the presence of complex carbohydrates on the surface of blood cells that elicit an immune response in laboratory testing or in the case of bodily exposure by means of a blood transfusion. Blood type is a fundamental biological characteristic of an individual that does not change during their lifetime.

In forensic work, a person's blood type can be ascertained from very small traces of blood found at a crime scene. It is often possible, therefore, to know a person's blood type early on in an investigation based on this testing. The ABO blood system is not a complex information system; there are only three basic blood types, each of which can be further designated as Rh positive or Rh negative, yielding a total of six major blood types.

In some cases, no blood is found by the investigators, but there may be saliva or other mucus-containing bodily fluid that can be identified. If the person from whom the bodily fluid originates carries the dominant secretor gene, that individual will secrete the ABO antigens in mucus, and it is possible to infer the blood type from these fluids. Secretion of soluble blood type antigens is now know to be a function of the alpha(1,2)fucosyltransferase gene. Careful study of this gene reveals that the secretor gene is present in the vast majority of people tested, but nonsecretors carry a nonsense mutation in both copies of their secretor gene, rendering them silent. Approximately 75%–80% of Caucasians carry at least one copy of the secretor gene, and they therefore, secrete soluble antigens into bodily fluids. The secretor phenotype also is a fundamental biological characteristic of a person that remains constant throughout life. This is another piece of biological evidence that can be collected in trying to match a sample found at a crime scene with potential suspects.

Blood typing and secretor analysis are not highly informative systems of information compared with existing DNA technologies such as STR analysis because of the limited numbers of different categories into which all people can be placed. Nevertheless, they played a significant role in the collection and analysis of forensic specimens prior to the advent of more informative DNA systems, and they continue to play a small role today. These kinds of evidence are far more persuasive as negative evidence (to rule out a suspect) than as positive evidence (to confirm a suspect) because many people can match by random chance.