Cellular and humoral immune deficiency are widely reported, but with a spectrum of clinical expression ranging from clinically-silent laboratory abnormalities (reduced CD4+, CD8+ T-lymphocytes, thymic emigrants, low percentage of naïve T-lymphocytes, increased memory T-lymphocytes, reduced TCRαβ/ TCRγδ ratio in peripheral blood) to clinically relevant immunodeficiency, particularly hypogammaglobulinaemia, which presents with recurrent, chronic respiratory tract infections. This evidence concerns the gene CD4 and immunodeficiency disease.