<prompt> The most common primary immunodeficiency is:Common variable immunodeficiencyIsolated IgA immunodeficiencyWiskott-Aldrich syndromeAIDS  <response>: Ans. is b i.e., Isolated IgA immunodeficiencyRef: Harrison, 18the, p. 2704 and 17the, p. 2058Selective IgA deficiency is the most common primary immunodeficiency, familial more common, naive B cells are not able to differentiate into IgA producing cells. Along with IgA, IgG2 and IgG4 may also be reduced. 