For instance, phage phi X174 specific to E. coli strains has been used for more than 30 years as an antigen to assess the humoral immune response in patients with primary and secondary immune deficiencies, such as severe combined immunodeficiency (SCID), X-linked agammaglobulinemia (XLA), X-linked hyper IgM syndrome, and Wiskott–Aldrich syndrome, as well as in bone-marrow recipients, people infected with HIV, and patients treated with immunosuppressants such as CTLA4-Ig [63]. Here, CTLA4 is linked to Bruton-type agammaglobulinemia.