The purpose of this Phase I/II study is to evaluate clinical response and safety following administration of recombinant human CD40 ligand (rhuCD40L) in up to 5 patients with X-linked hyper IgM syndrome (XHIM). XHIM is a rare genetic disease caused by mutations in the gene encoding CD40 ligand (CD154) and is characterized by hypogammaglobulinemia, opportunistic infections, and an increased risk of neoplastic disease. Despite treatment with intravenous gamma globulin, the expected survival of patients with XHIM is less than 20% by the age of 25. The proposed protocol is a proof of principle study designed to determine if administration of rhuCD40L can reverse the core immunologic defects of patients with XHIM. To this end, we will immunize patients with neo antigens, specifically keyhole limpet hemocyanin (KLH) and Bacteriophage Phi-X 174 (PhiX174) to evaluate antigen-specific B and T cell responses. Clinical response and toxicity will be evaluated using routine hematological and clinical evaluation, quantitation of KLH and PhiX174 specific IgG in serum, measurement of proliferation and cytokine production to KLH simulation in vitro, and FACS analysis to quantitate memory B and T cells. Our long-term goal is to define a therapeutic regimen that will provide effective immunological reconstitution to patients with XHIM and improve life expectancy.