Recently, gut inflammation complicating functional hypogammaglobulinemia due to CVID and selective IgG subclass deficiency has been characterized as a T helper type 1 (Th1) inflammatory response, with excess IL-12 cytokine production associated with diarrhea and weight loss as well as reduced D-xylose absorption and steatorrhea. This protocol aims to test specific anti-IL-12 therapy in this patient group. It has been previously shown that therapy targeted to IL-12 successfully treated the Th1 gut inflammation of Crohn s disease (CD). Ustekinumab, a monoclonal antibody to the p40 subunit of IL-12 and IL-23, is FDA approved for the treatment of moderate to severe plaque psoriasis, active psoriatic arthritis, and more recently, moderately to severely active CD. This protocol is designed to measure the safety of ustekinumab in patients with functional hypogammaglobulinemia and CVID enteropathy, as well as measure effects on symptoms, gut function, expression of immune cell surface markers, production of cytokines and global gene expression from blood and gut mucosal mononuclear cells, and the gut microbiota.