Dear Dr. Anita Ma

It is known that the process of formation of antibodies against foreign antigens, occurs after the presentation of these antigens on macrophages. However, antibodies to the blood group antigens are formed spontaneously during the first few months of life. Concentration of the blood group antigens in the erythrocytes of newborns can reach 50% of the level of an adult person. With age, Ag A and Ag B become more and more branched.

With respect to antibodies, in newborns up to 4 months old, the titre of Ab A and Ab B is low. The number of Ab A and Ab B increases during the first year in humans. The titre of Ab A and Ab B becomes maximum at the age of 10–20 years, and then, especially in old age, their titre decreases. The naturally occurring anti-A and anti-B antibodies in the serum consist mainly of immunoglobulin (Ig) M and, to a lesser extent, IgG.

Several hypotheses have been proposed regarding the antibodies.

1. Ab A and Ab B are so–called natural antibodies (Abs).

2. Bacteria could move from the lumen of the intestine into the blood and since they have polysaccharides similar to Ag A and Ag B, Ab A and Ab B arise.

3. Viruses carry polysaccharides on their membrane and then they are presented through the auto-presentation system.

4. Glycosylation errors occur due to the overload of enterocytes with lipids.

5. The de-sialylation of polysaccharides occurs due to the delay of post-Golgi carriers at the endosome level due to the enterocyte overload with lipids.

For more information regarding the mechanisms of formation of antibodies against blood group antigens, you may want to refer to the article attached below.

Article Mechanisms of Formation of Antibodies against Blood Group An...

To summarize:

The ABO blood group system is based on the type of sugars expressed on the surface of RBCs. In case of a person with type A blood, he/she is not born with anti-B antibodies, but the person makes these antibodies by the time he/she completes first year of life. The reason being that the molecules (antigen) of the ABO blood group system are oligosaccharide chains, which are attached to lipids and proteins that are anchored in the RBC membrane, which are not unique. Similar type of sugar molecules is present in bacteria and viruses which the person may get exposed to.

Thus, one does get exposed to other antigens of the ABO blood group system which is not through blood transfusion.

Regards,

Malcolm Nobre

Malcolm Nobre Thank you so much for the answer which makes big sense to me.