The T helper cells (T_h cells), also known as CD4⁺ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considered essential in B cell antibody class switching, breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils. CD4⁺ cells are mature T_h cells that express the surface protein CD4. Genetic variation in regulatory elements expressed by CD4⁺ cells determines susceptibility to a broad class of autoimmune diseases.