For most patients, rhuCD40L will be administered by injection under the skin over a period of six months and follow-up exams are required at 2-month intervals for an additional 6 months. During the study, patients will be maintained on intravenous gamma globulin, antibiotics to protect against opportunistic infection, and, if needed, growth factors to control neutropenia. The immune response to rhuCD40Lwill be measured by routine methods such as measuring a patient's ability to synthesize gamma globulin when challenged with immunizations to keyhole limpet hemocyanin (KLH) and Bacteriophage Phi-X 174 (Phi-X 174). Our long-term goal is to define a therapeutic regimen that will provide effective immunological reconstitution to patients with XHIM and improve their life expectancy.