Antigens are foreign substances which when introduced into the body can provoke the formation of specific antibodies. Antigens and their specific antibodies react with each other when they come in contact.
In blood banking, red cell antigens are of main importance. White cells, platelets and biological fluids are also known to exhibit antigenic properties. RBC antigens are fixed protein or lipoprotein structures incorporated in the lipid membrane of the RBC.
Formations of human RBC antigens are coded by specific nuclear DNA loci and composition known as genes. Since RBC antigens are DNA specific, if one is born with an antigen, he has it for life. More than 300 antigens have been discovered on the surface of RBC.
Associated inherited variations of allied genes gives rise to the classification of antigens into associated blood groups, i.e. ABO, Rh, Kell, Kidd, Duffy, MNS and others.
Red Cell Antigens:
High frequency antigens (Public antigens):
Antigens with a frequency of >99% on human red cells, e.g. Kph, Jsh, Lub, k, vel, Ge, etc.
Low frequency antigens (private antigens):
Antigens with a frequency of <1% of human red cells, e.g. SW9, Js9, Kpa, etc.
Antibodies are produced by circulating lymphocytes and plasma cells (immunocytes) in an individual following antigenic stimulation. The antibodies are immunoglobulins, present in serum and can be demonstrated serologically by reacting with corresponding antigen (immunologic reaction).
The antibodies are mainly of two types – Natural antibodies and Immune antibodies. The natural antibodies do not penetrate from mother to foetus and thus protects it from incompatible maternal blood.
The natural antibodies can be present naturally in an individual from birth, without any antigenic stimulation. These antibodies are generally of IgM type and with high molecular weight.
Immune antibodies are produced when the red cells carrying corresponding antigens enter into an individual, who normally lacks the antigen (e.g. anti – D). These are usually IgG type antibodies and react best at body temperature. These have low molecular weight.
Immune antibodies produced by infectious agents do not interfere normally in blood banking procedure the exceptions are like mycoplasma and some of the bacterial infections.
Antibodies stay in the plasma and form a part of humoral system of the body. Hence, for the identification of the antibody, clotted blood, which yields the serum, is required. Most often the antibodies react specifically with its corresponding antigen that stimulated its production (e.g. D antigen with Anti D).
Occasionally however, two antigens have certain chemical groups in common and an antibody made against one of them will react to some degree with the other. This apparent dual specificity is known as cross-reactivity.
Isoantibodies are produced in the same species as the antigen source. Anti-D (anti-Rh) produced by Rh negative individual, after receiving Rh positive cell is an example of isoantibody production.
Hetroantibodies however, originate in species other than the source of antigen. The antihuman globulin used in the antihuman globulin reaction (Coomb’s reaction) is an example of hetroantibody.
Reactions in Immunohematology:
i. The red cell antigen involved in the haemagglutination is referred to as agglutinogen.
ii. The antibody involved in the haemagglutination is referred to as agglutinin.
iii. The natural antibodies such as anti-A and Anti-B react with corresponding agglutinogens A and B in the saline medium at room temperature.
iv. The immune antibodies such as anti-D react only in albumin solution with incubation at 37°C.
v. The haemagglutination reaction first results in coating of the antibody (adsorption) on the red cells, followed by bridging of the red cells and resulting agglutination.
vi. Some of the immune antibodies may not produce haemagglutination but remain adsorbed on the red cells carrying corresponding antigens. This is called sensitization. These cells can be later detected by their reaction with antihuman globulin reagent.
vii. Visible haemagglutination reactions require proper proportion of antigen and antibody.