<prompt> The commonest primary immunodeficiency isCommon variable immunodeficiencyIsolated IgA immunodeficiencyWiskott-Aldrich syndromeAIDS  <response>: Isolated IgA deficiency Extremely low levels of serum secretory IgA may be IgG2 IgG4 too Defective differentiation of B cells to IgA producing plasma cells CF Mucosal immunity affected Recurrent sinopulmonary infections diarrhoea When transfused with blood containing IgA may develop fatal anaphylactic reactions 