Multiple choice Deletions in the T-cell CD154 (CD40L) gene produces which of the following? Congenital X-linked agammaglobulinemia IgA deficiency Wiskott–Aldrich Syndrome Deficiency in cytotoxic T-cell activity The hyper–IgM syndrome Reveal answer Fill a bubble to check yourself E Correct answer Explanation Lack of CD154 (CD40L) prevents signalling to the B-cell CD40 to bring about Ig class-switching away from IgM.