Isoantibodies

Isoantibodies are antibodies (proteins that defend against foreign agents, such as bacteria) in blood.

Hemagglutination (clumping of red blood cells) reactions are used in the typing of blood. The presence or absence of antigens (a substance perceived as foreign to the body), designated A and B, located on the surface of red blood cells is determined by employing the use of specific antisera (antibodies). Since an individual possesses antibodies to the opposite antigen, when anti-A antiserum is mixed with type B red blood cells, no hemagglutination will occur. Similarly, persons of blood type A will have anti-B in their serum. Persons with type AB blood contain both A and B antigens on their red blood cells. Individuals with type O blood lack both A and B antigens, but they do have anti-A and anti-B in their serum.

ABO blood types are controlled by three alleles (versions of genes): IA and IB are co-dominant and both are dominant over the recessive i. The homozygous recessive condition (ii) results in type O blood. A person with blood type A may have either of the following genotypes: IAIA or IAi. The genotype IAIB results in type AB blood.

The Rh factor is a complex of over 30 different antigens on the surface of human red blood cells. The Rh factor that is routinely used in blood typing is the called the Rh, or D, antigen. The presence of the Rh factor is determined by a hemagglutination reaction between anti-Rh antiserum and red blood cells. Persons who have the Rh antigen are called Rh-positive. Rh-negative individuals do not naturally have anti-Rh in their sera.

Isoantibodies are present in human serum. An individual possesses isoantibodies to the opposite A or B isoantigen. For example, persons of blood type A will have serum containing isoantibodies to the B antigen. Rh-negative individuals do not normally have anti-Rh antibodies in their sera. When red blood cells with Rh antigen are introduced into Rh-negative individuals, anti-Rh-antibodies are produced.

In 1929, Kosaku Yosida established the existence of isoantibodies in body fluids other than blood (saliva, sinus secretions, etc.). By establishing the existence of serological isoantibodies in fluids other than blood, Yosida paved the way for far more sophistication in the forensic analysis of bodily fluids, along with the ability to use multiple means of identification of a single perpetrator of a crime.