MHC class II deficiency is inherited as an autosomal recessive trait and involves a defect in the coordination of transcription factors involved in regulating the expression of all MHC class II genes (HLA-DP, HLA-DQ, and HLA-DR)

Question

A. What is the effect of MHC class II deficiency?
B. Explain why hypogammaglobulinemia is associated with this deficiency.

What will be an ideal response?

Answer 1

A. MHC class II deficiency affects the development of CD4 T cells in the thymus. If the thymic epithelium lacks MHC class II, positive selection of CD4 T cells will not take place. CD8 T cells are not affected because MHC class I expression is unaffected by this defect.
B. To produce antibody, B cells require T-cell help in the form of cytokines produced by CD4 TH2 cells. Low immunoglobulin levels (hypogammaglobulinemia) in MHC class II deficiency are attributed to the inability of B cells to proliferate and differentiate into plasma cells in the absence of TH2 cytokines.