Combined Immunodeficiency Disorders

Combined immunodeficiency disorders (CID) affects both T cell and B cell and can lead to a spectrum of associated abnormalities. In this chapter, we use case-based discussion to describe two prototypes of combined immunodeficiency disorders: X-linked hyper-IgM (XHIGM) and Wiskott-Aldrich Syndrome (WAS). XHIGM, the most common form of hyper-IgM syndrome, is caused by mutations in the CD40LG gene encoding the CD40 ligand protein. The CD40 ligand protein is expressed on T cells and is required for immunoglobulin class switch recombination. XHIGM patients are susceptible to Pneumocystis jirovecii pneumonia and Cryptosporidium sp. induced diarrhea. Hepatobiliary disease, neutropenia, and autoimmune disease can complicate the clinical course. Wiskott-Aldrich syndrome (WAS) is an X-linked disease caused by the WAS gene mutation leading to a defective WAS protein (WASp). The WASp is involved in actin polymerization, cytoskeletal rearrangement, and cell signaling in non-erythroid hematopoietic cells. The level of WASp expression leads to a range of clinical phenotypes. Patients typically present with microthrombocytopenia, eczema, and immunodeficiency. The mainstay treatment for both XHIGM and WAS relies on infection prevention with immunoglobulin replacement therapy and antimicrobials. Hematopoietic stem cell transplantation and possibly gene therapy can be corrective options.