According to this hypothesis, exposure to some pathogens during infancy and youth benefits
individuals by stimulating immune responses and establishing a healthy balance of T-cell subset
activities so that no one response dominates. For instance, the immune system of newborn babies
may be biased in the TH2 direction by the uterine environment. TH1-TH2 balance may be
restored by the occurrence of infections in the developing neonate. However, in the sanitary
conditions promoted by Western medicine, the neonatal immune system may not have the
exposure to infections that would otherwise reorient it to generate TH1-type responses. Evidence
that pathogen exposure induces NK-mediated interferon γ secretion, which biases the responses
of the subject away from the TH2 direction and thus away from antibody production that
contributes to asthma and other allergies, also supports this view.
Note that the H antigen is present in all blood types.
Most adults possess IgM antibodies to those members of theABH family they do not express.
This is because common microorganisms express carbohydrate antigens very similar in structure
to the carbohydrates of theABH system and induce a B-cell response. B cells generating
antibodies specific for theABH antigens expressed by the host, however, undergo negative
selection. For example, an individual with blood typeArecognizes B-like epitopes on
microorganisms and produces antibodies to the B-like epitopes. This same individual does not
respond toA-like epitopes on the same microorganisms because they have been tolerized to self-
Aepitopes. The clinical manifestations of transfusion reactions result from massive intravascular
hemolysis of the transfused red blood cells by antibody plus complement (complement mediated
lysis triggered by IgM).