Increased susceptibility to infection, and diagnosis after 4th year of life and undetectable serum IgA (when measured with nephelometry <0.07 g/L) but normal serum IgG and IgM (measured at least twice) and secondary causes of hypogammaglobulinemia have been excluded and normal IgG antibody response to all vaccinations and exclusion of T cell defect. Here, CD40LG is linked to agammaglobulinemia.