X-linked infantile hypogammaglobulinemia a congenital, primary immunodeficiency characterized by decreased numbers (or absence) of circulating B lymphocytes with corresponding decrease in immunoglobulins of the five classes; associated with marked susceptibility to infection by pyogenic bacteria (notably, pneumococci and Haemophilus influenzae) beginning after loss of maternal antibodies; X-linked recessive inheritance caused by mutation in the Bruton tyrosine kinase gene (BTK) on Xq.