Within the memory compartment, switched memory B cells differentiate into IgG, IgA or IgE-producing plasma cells; unswitched or marginal-zone like B cells differentiate into IgM-producing plasma cells and are important for anti-polysaccharide responses, and “IgMonly” memory B cells have been found to increase in some disorders such as hyper-IgM syndrome [21]. This evidence concerns the gene CD40LG and hyper-IgM syndrome.