In all forms of Hyper-IgM (HIGM) syndrome, the normal process of immunoglobulin heavy chain class switching from IgM to other classes fails. Patients have low to absent serum IgG, IgA and IgE with normal to elevated levels of IgM. Hyper-IgM syndrome Type 1, caused by mutations in the CD40LG gene, is by far the most common type, accounting for approximately 65-70% of HIGM cases. Type 2 (caused by mutations in AICDA) accounts for likely less than 5% of cases, while Type 3 (caused by mutations in CD40) and Type 5 (caused by mutations in UNG) account for only a small number of cases.
Hyper-IgM syndrome Types 1 and 3 are clinical similar, with patients displaying a more severe phenotype compared to Types 2 and 5. These individuals often present with severe, recurrent sinopulmonary infections, Pneumocystis jeroveci (aka Pneumocystis carinii) pneumonia (PCP), chronic diarrhea and may have intermittent or persistent neutropenia. A distinguishing feature of Types 1 and 3 is that individuals are susceptible to opportunistic infections, which is typically not observed in individuals with Types 2 or 5. In addition, individuals with Types 1 or 3 often present at an earlier age (within the first one or two years or life) as compared to individuals with Types 2 or 5, where age of onset can vary from as early as the first few years of life to even as late as the second decade.
Hyper-IgM syndrome Types 2 and 5 are also clinically similar. These individuals typically have a milder disease presentation; the most common features observed are a susceptibility to bacterial infections and lymphoid hyperplasia. One important biochemical difference between these two types is that individuals with Type 5 have normal rates of somatic hypermutation (SHM), while individuals with Type 2 typically do not have intact SHM (although mutations in a specific region of the AICDA gene can result in normal SHM frequencies).