How the specific antibody against a antigen is their in the b blood group individual without exposure to the a antigen?
Answers
If I am correct, your question is why ABO blood group reciprocal antibodies present in the human being (A group person has anti B, B group person has anti A, O group person has anti A and anti B, AB group has no ABO antibodies in the serum) while other blood group system like Rh, Duffy, Kell, Kidd, etc are restricted on the red blood cells and acquired antibodies (IgG) can only be present after the exposure of corresponding antigen which is deprived on the RBC of a person (antigenicity of these blood group systems antigens are of protein based.)There is no evidence of these antibodies are present in a person without exposures of these antigens.
ABO blood group system is of clinically prime important blood group system because of the fact that
1. Antigens of this blood group system are carbohydrate (for A antigen it is NAC galactosamin and for B it is galactosamin)
2. Reciprocal antibodies (IgM) are present in the serum of a person and causes intravascular hemolysis.
3. Formation of ABO group antigens are not directly under control of ABO genes but genes coded the information via transferase enzymes.
Now your question is how it is appears in a person? These ABO carbohydrate antigens are also present on the cell membrane of saprophytic bacteria (commensals) which appears in the gut in neonatal period and on the cell wall of the pollen grains. Now developing immune system of the neonate treat them non self to those ABO antigens which are absent on the RBCs and develop the antibodies in the serum of a person. Now, why these antibodies are IgM? It is because that there is a continuous, sustained and subclinical stimulus throughout the life so these are IgM (there is no primary and secondary stimuli).
Now, these antibodies are against the bacteria and pollen grain antigens ( against the Natural occurring antigens and also called the natural occurring antibodies ) and equally reacting with human antigens. It is a good example of cross reacting antibodies