mani
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5yr old boy, recurrent ear infections receives booster immunization against DPT. theres below normal humoral response to tetanus toxoid(tt). peripheral B& t lymphocyte counts are normal. He makes positive antibody in both passive hemaglutination & complement mediated lysis of tt-coated erythrocytes. Antibodies do not opsonize tt coated latex particles for phagocyosis & do not precipitate tt efficiently. defect is
a. affinity maturation of Ig
b. Ig isotype switching
c. somatic mutation of Ig genes
d. recombination heavy chain genes
e. recombination light chain variable region genes
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| tonguo
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c. the mutation is at the CH3 region of gamma gene.
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| mani
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would u please explain it?
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| tonguo
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a and e r both talking about the epitope binding region of Ig, which is functional as shown by positive hemagglutination and complement fixation. b. if there is a defect in isotype switching, complement-fixing Ab should be negative or at least abnormal. I guess d is taking about VDJ recombination, which actually doesn't affect heavy chain struc or func. since the Ab binds complement, its CH2 is normal, so it has to be the CH3 region whats problematic. hope this make things clear for u. btw, this is a very good q that definitely requries thinking.
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| mani
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thanx a lot. there are 2 points.
regarding option d, i think it is concerned with with both constant and variable regions of heavy chain.
and 2nd: can tell me functions of CH1-4 region functions separately.
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