Introduction:
• Each species of animals , including humans has certain antigens on the surface of the red cells which are unique for that species.
• These are known as isoantigens. Similarly certain antigens , the alloantigens are common to some, but not all members of that particular species.
• Blood group serology involves the detection of these antigens & their antibodies.
Blood group genteics:
• All the antigenic substances present on RBC of indiviual are inherited. The unit of inheritance is a gene & each antigen is controlled by gene for that antigen.
• The position of each gene on a chromosome is called its locus.
• Some terminology:
• Allele or allelomorph: Two or more genes responsible for same chracteristics occupying the same position on the chromosomes.
• Homozygous: A person who has inherited same allelic genes from both parents eg: having both blue eyed colour genes.
• Hetrogygous: A person who has inherited different allelic genes for trait. eg: having a blue & brown eye colour genes.
Dominat gene: Certain alleles are stronger than other alleles. eg: brown eyes genes mask the presence of blue eye genes, & are therefore dominant over blue eye genes. A dominat gene expresses itself in both homozygous & hetrozygous states.
• Recessive gene: They are expressed only when dominant allele is absent eg: in Homogygous state. Colour of eye is blue ,if both alleles are for blue eyes.
• Co-dominat gene: In this ,various alleles for the same character are equally dominant eg: alleles for a particular blood group system.
• Genotype: The genetic make up of an individual or cell is called its genotype.
• Phenotype: It is the observable characteristics of individual or a cell eg: an individual of group Bo genotype would be expressed as group B because O gene is rcessive.
Blood Group Antigens:
• The blood group antigens include substances on the RBC & on other consitutents such as leucocytes, platelets & plasma.
• The blood consitutent antigens which are governed by single gene or a group of genes are inherited as groups.
• Blood groups controlled by set of genes are called a blood group system.
• Most blood group antigens follow the law of inheritance. Chemically blood group antigens are usually glycoproteins, lipoproteins or glycolipids in nature.
Blood Group Antibodies:
• Specific blood antibodies develop in response to antigenic stimulation by particular blood group antigen.
• The level of antibody activity depends on the immunogenicity of corresponding antigen.
• Alloantidodies:
• Are antibodies which are present in some members of species but not all. eg: anti-A antibodies are present in all
humans beigns belonging to blood B & O.
• Due to the presence of alloantibodies, it is necessary to select a blood donor who is negative for corresponding antigen,i.e if recipient has anti-A antibodies, he cannot recieve blood which contain group A antigen.
• They are of three type:
1. Naturally occurring: The antigenic stimulus is unknown. ABO antibodies belong to this type.
2. As a result of immunization through transfusion, some red cell antigens which are absent in the recipent may be introduced & induce antibody production.
3. Induced by exposure to foetal erythrocytes either during pregneancy or during delivery. These antibodies are similar to those produced by blood transfusion. Development of Rh antibodies in a Rh negative mother by a Rh positive foetus is good example.
Autoantibodies:
• An autoantibody is the antibody which is induced by an antigen in same individual. It also reacts with same antigen if it present in other individual.
• In some cases the reaction of antigen & its autoantibody may not show any demonstratable clinical symptoms, whereas sometimes such a reaction may lead to haemolytic anaemia, leucopenia or thrombocytopenia.
• It can be divided into two general catagories depending on their optimum reacting Temperature.
1. Warm autoantibodies: These consitutue about 85% of autoantibodies. Their optimal temperature is 370C. They are generally IgG in nature & can be detected with direct antibody test.
2. Cold auto antibodies: About 15% are of this type. they are generally IgM in nature & agglutinate RBC strongly at 40C weakly at 240C & not at all 370C. Most of are present in normal individual, very few are associated with diseases. It is important to be aware of cold antibodies, when the patients body temperature needs to be lowered for procedures such as cardiopulmonary bypass.
: Main Blood group system:
• Approximately 600 blood group antigens have been describes so far.
• The konwoledge of blood group antigens is important during blood transfusion.
• Among these ABO system is most important blood group system in blood transfusion & in organ transplantation because of two unique features:
1. Strongly reactive antibodies are present in sera of individual who lack the corresponding antigens:
2. A & B antigens are present on many tissues cells in addition to red cells.
• The ABO system consist of four blood groups or phenotypes : A, B, AB & O. The two antigen A & B are responsible for these four groups.
• If A antigen is present on red cell individual is said to belong to A group ,If B antigen is present on red cell individual is said to belong to B group, & If AB antigen is present on red cell individual is said to belong to AB group.
- While O groups have neither A or B antigen on their red cell.
• Three allelic genes A, B & O,can be inherited in ABO system. The following combination of alleles is
possible: AA, AO, AB, BB, BO, OO resulting in A, A, AB, B, B & O group individuals respectively.
• This so because A & B genes are dominat & O gene is recessive.
Law of Inheritance of ABO Groups:
• According to Bernstein's theory two laws of inheritance have been proved.
1. It is not possible that an offspring can possess antigen A, B or both unless it is inherited from one or both parents.
2. A parent of blood group AB cannot produce an offspring of group O. Similarly a parent of blood group O cannot produce AB offspring. The reason for this law is that group AB is hetrozygygous & therefore A & B genes must come from two parents.
Group O individual have antigen called H antigen on the surface of red cells , which is precusssor of A & B antigen.
Subgroups of ABO:
• Both groups A & B can be further subdivided . The most important of these subgroups are A1, A2. Both A1 & A2 cells react with anti A antiserum.
• Approximately 20% of group A & AB persons belong to subgroup A2 & 80% to A1. Several subgroups of B have been identified but they are very rare.
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