Thus the clinical features of ICOS deficiency parallel those of another T-B cell costimulatory defect, CD40 ligand deficiency, in which patients not only show hypogammaglobulinemia but also evidence of an accompanying T-cell immunodeficiency, commonly in the form of Pneumocystis or Cryptosporidium infections [18]. Here, CD40LG is linked to T-cell immunodeficiency.