BLOOD AND IMMUNOGENETICS

The genetics of the immune response is termed as immunogentics . This branch of science deals between relationship of the immune system and genetics. The study of the Rh, ABO and other blood groups or the HLA system is important to kidney and other transplants.

Use in immunohematology:
• Paternity testing
• Phenotyping/genotyping blood donors and transfusion recipients

Autoimmune diseases, such as type 1 diabetes, arthritis are complex genetic traits which result from defects in the immune system. Identification of genes causing the immune defects may be targeted for therapeutic advance. Besides genetic variations can also help to define the immunological pathway leading to disease.

It's relatively a new Science. Its origin is usually attributed to Edward Jenner who discovered in 1796 that cowpox, or vaccinia, induced protection against human smallpox. It took two centuries before the World Health Organization announce in 1979 that smallpox had been eradicated.

BLOOD

The human blood is a fluid connective tissue containing two principle components: cells which include red, white and platelets and the liquid part which is called plasma. The plasma is composing of clotting protein fibrinogen and serum. When, blood of the individuals is mixed with the serum of the other person, then in certain cases clumping of red blood cells takes place but, in other cases do not. The clumping or the agglutination of the red blood cells takes place due to the antigen antibody reaction which was first discovered by Landstenier in 1900.

A blood group system consists of one or more antigens controlled at a single gene locus, or by two or more very closely linked homologous genes with little or no observable recombination between them.

ANTIGEN - ANTIBODY REACTION

Whenever any foreign micromolecule or macromolecule is injected in the blood of man and other higher vertebrates, then the blood will react to eliminate or neutralize or immunize that foreign substance from it. Such foreign substances are called agglutinogens or antigens. The antigen may be plant or animal proteins or bacterial or viral toxins. The response to the appearance of such antigen in the blood is the elaboration or production of another protein molecule which will superficially combine or interact with the antigen in the same way. The second protein which is produced in the blood to immune an antigen is called immune body or agglutinins or antibody. The antibodies are produced by the modifications of the molecule of gamma globulin proteins which are synthesizes by plasma cells. The interaction between antibody and antigen changes the form of the antigen in some way. So that, it can be destroyed, inactivated, phagocytized or otherwise eliminated from blood circulation.


The antibodies are highly specific for a particular antigen and they may be of the following types:

1. Acquired antibody: certain types of antibodies are produced by plasma cells only during the time of entry of foreign antigenic proteins in the blood stream. Such antibodies are called acquired antibodies.
2. Natural antibody: In few cases, the antibodies are produce naturally and normally by the blood, even in the absence of appropriate antigens. Such antibodies are called natural antibodies. The antibodies which are involved in A-B-AB-O groups of man are natural antibodies.


A,B,AB AND O BLOOD GROUPS

There are two kinds of agglutinogens or antigens, called A and B antigens from the surface of red blood cells of human blood. He found that out of A and B antigens, a person mate contain either one or neither of them. Accordingly, he recognized three kinds of blood types or bold groups: type A, type B. type O.The forth and the rarest, the AB blood group type. For A and B antigens, there occur two agglutinins or antibodies: anti -A or alpha and anti -B or beta. All the four have different agglutination properties.

Individuals who are AB blood types have inherited an A and B blood group gene from both parents.

Many of the blood group antigens are useful in determining exclusion of paternity and probability of paternity. For example, if a child is type A and the mother and father are type O, it is impossible that the child is theirs.

POPULATION GENETICS

Population genetics is important in paternity testing and also in forecasting the likelihood of finding compatible blood for a person with multiple antibodies in their serum. If a person has antibodies (in their serum) toward a certain blood group antigen, they should only receive red cells that do not have that antigen to avoid a transfusion reaction.
The frequency of blood group phenotypes is determined from a large, random population of the same race. This is because some antigens are more frequent in particular races. If a person has 1 antibody present, then only cells negative for that antigen are given. For example: 60% of the population has the C antigen, meaning 40% or 0.40 do not have the antigen (must total 100%). So, 40 out of 100 units of blood will not have the C antigen. This an a quantitative science

If a person has more than one antibody present, then a slightly different calculation is used to determine how many units must be screened to find a compatible unit for the patient.

Example:

30% (0.3) are negative for C antigen
91% (0.91) are negative for K antigen
33% (0.33) are negative for Fya antigen

To find the percentage of the population negative for all three antigens, multiply the frequencies:

0.3 X 0.91 X 0.33 = 0.09 (9%)

What does this mean? 9% or 9 out of 100 units of blood would possibly be negative for all three antigens listed. Since it would be very time consuming to test that many units, it may be necessary to contact a local blood donor center for the units.

Other calculations

Hardy-Weinberg equilibrium for genotype frequencies is:

p2 + 2pq + q2 = 1

p = frequency of dominant gene
q = frequency of recessive gene
p2 = frequency of dominant (homozygous) genotype
2pq = frequency of heterozygous genotype
q2 = frequency of recessive genotype

The gene frequency of alleles is calculated using the following formula:

p + q = 1

THE RH BLOOD TYPE AND ITS INHERITANCE.

Besides A and B antigens , another antigen called Rh factor was reported from erythrocytes of certain persons.Rh factor or Rhesus system is written as either positive (present) or negative (absent). Most people are Rh positive. This factor does not affect your health except during pregnancy for women.

A woman is at risk when she has a negative Rh factor and her partner has a positive Rh factor. This blend can produce a child who is Rh positive. While the mother's and baby's blood systems are separate, but there are times when the blood from the baby can go into into the mother's system. This causes the mother to create antibodies against the Rh factor, thus treating an Rh positive baby like an intruder in her body. If this happens the mother is said to be sensitized.
A sensitized mother's body will make antibodies. These antibodies will then attack an Rh positive baby's blood, which breaks down the red blood cells of the baby and make the baby anemic. Severe cases this hemolytic disease can cause illness, brain damage and even death.
Sensitization can also occur during a blood transfusion, miscarriage, abortion, ectopic pregnancy and even during some procedures, like amniocentesis. Since the antibodies do not disappear and rarely cause a problem in first pregnancies, it is very important to be screened thoroughly to prevent such things during second pregnancy.

Hemolytic disease can be prevented for many women, if they are not already sensitized. Rh immunoglobulin (RhIg) is a blood product given by means of injection to help the Rh negative mother by "minimizing her reaction to the Rh positive red cells. Reactions to the medication are generally minor, including soreness at the injection sight and sometimes a slight fever.
Since a small number of unsensitized women may have problems with the end of pregnancy, many practitioners recommend that she be given an injection of RhIg (also known as Rhogam) at 28 weeks gestation, to prevent the few cases of sensitization that occur at the end of pregnancy. Each dose of RhIg lasts about 12 weeks. The mother will also be given RhIg within 72 hours of birth if the child is Rh positive. The baby's blood type can be determined easily after birth by cord blood samples.

RhIg may also be given after an amniocentesis, miscarriage, abortion or postpartum sterilization (tubal ligation).

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