Selective IgA deficiency may be defined as an inborn state characterized by a decrease of serum IgA levels below 8 IU/1 (approximately 5 mg/dl) which may be associated with clinical symptoms of disease. The frequency of this condition in the general population varies between 1 : 400 and 1 : 3000 in different countries. Patients with defects of chromosome 18, ataxia teleangiectatica and with connatal rubella syndrome have a high incidence of IgA deficiency. Inspite of the decrease in circulating IgA there are B-lymphocytes containing IgA molecules in the peripheral blood. Thus it has been concluded that transformation of B-lymphocytes into IgA bearing plasmacells is stunted by another mechanism. While small amounts of IgA may be released by transformed plasmacells the capacity of B-lymphocytes to mature into fully functioning plasmacells releasing normal amounts of IgA is defective. T-cells acting as suppressor cells for IgA differentiation have been demonstrated in peripheral blood and are a possible explanation for this phenomenon. The majority of individuals with IgA deficiency are healthy. Evaluations of increased susceptibility for infections have to consider the fact that 6 respiratory tract infections per year are the average for any preschool child. However a number of children with IgA deficiency suffer from recurrent bacterial infections such as sinusitis, bronchitis and pneumonia, usually responding well to antibiotic treatment. IgA deficiency has an established correlation with atopic disease. There is an 40 fold increase in incidence of allergies and autoimmune diseases such as rheumatoid arthritis, lupus erythematodes and thyroiditis in individuals with IgA deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)