CD40LG and infection: In fact, as a natural consequence of infection, the immune system initiates a multitude of responses, one of which is, besides the permanent presence of IgA antibodies both at the epithelial–mucosal barrier and in the blood, the production and secretion into the bloodstream (as part of the humoral immune system) of IgM antibodies in response to viral invasion of the interior of the body (i.e. the event of infection).