Hyper IgM Syndromes is a group of primary immune deficiency disorders characterized by defective CD40 signaling by B cells affecting class switch recombination and somatic hypermutation. Immunoglobulin (Ig) class switch recombination deficiencies (CSR-Ds, which were previously named "dysgammaglobulinemia" and then "hyper-IgM syndromes") are characterized by elevated (or sometimes normal) serum Immunoglobulin M (IgM) levels and a considerable decrease in (or the absence of) Immunoglobulin G (IgG), Immunoglobulin A (IgA) and Immunoglobulin E (IgE) – suggesting defective CSR. As a consequence, patients with HIGM have decreased concentrations of serum IgG and IgA and normal or elevated IgM, leading to increased susceptibility to infections. The most common HIGM syndrome is X-linked and due to mutations of CD40 ligand (CD40L) expressed by activated CD4+ T lymphocytes.