It is largely established that in selective IgA deficiency (SIgAD), which is the most common pediatric antibody deficiency with incidence rates varying between 1:143 and 1:18,500 (18), the serum concentration is always less than 7 mg/dL associated with normal serum IgG and IgM levels. This evidence concerns the gene CD40LG and selective IgA deficiency disease.