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A 5 yrs old boy with recurrent ear infection receives his booster immunization against DPT.his humoral immune response to tetanus toxoid is well below normal.Peripheral B & T lymphocyte count and function are within normal range.Antibody that he makes is positive for both hemagglutination and complement mediated lysis.His antibodies do not opsonize toxoid coated latex particles for phagocytosis and do not directly precipitate it efficiently .What defect is present in this child ?

A; Affinity maturation of immunoglobulins
b: Immunoglobulin isotype switching
c: Recombination of heavy chain variable region
d Recombination of light chain variable region
e: somatic mutation of immunoglobulin genes
BB
Immunoglobulin isotype switching
Thank you for the question.