Previous studies have documented hypogammaglobulinemia, reduced memory B cell numbers and impaired responses to vaccination as common features of CD19-deficient patients; however, there is substantial variance in the severity of symptoms and ex vivo CD19-deficient B cell function when tested using either total peripheral blood B cells or a purified naïve population [17–21, 34, 35]. Here, CD19 is linked to agammaglobulinemia.