Isotype switching is mediated through two important protein interactions: CD40 on the B cells interacts with CD40L on activated T cells (IL-4 induced) to stimulate cells to swich from IgM molecules to other isotypes. Deficiencies in either molecule lead to severe immunodeficiency states with only IgM produced but no IgG or IgA antibodies. This syndrome is called the hyper-IgM syndrome, and in this case of CD40L deficiency, it is an X-linked immunodeficiency.
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