CD40LG and immunodeficiency disease: Therefore, loss of AID (encoded by the gene AICDA) leads to an inability for B cells to undergo CSR, which results in hyper‐IgM syndrome type 2 (HIGM2), a rare immunodeficiency characterized by normal or elevated serum IgM levels with absence of IgG, IgA and IgE, resulting in a profound susceptibility to bacterial infections.