CD4 and rheumatoid arthritis: On the other hand, CD4+ Th9 cells are involved in hypersensitivity and allergic reactions, as well as helminth infections, mostly secreting IL-9 (25), while CD4+ Th17 cells are generally found in digestive and respiratory mucous membranes where they release cytokines such as IL-17A, IL-17F, IL-22, and IL-6 that help establish a pro-inflammatory microenvironment which deregulation has been linked to autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, or asthma (26).