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York University
BIOL 3120

14. 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. 15. 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). He
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