<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2007) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2007) (Learn how and when to remove this template message) </Td> </Tr> <P> The T helper cells (T cells) are a type of T cell that play an important role in the immune system, particularly in the adaptive immune system . They help the activity of other immune cells by releasing T cell cytokines . These cells help suppress or regulate immune responses . They are essential in B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages . </P> <P> Mature T cells express the surface protein CD4 and are referred to as CD4 T cells . Such CD4 T cells are generally treated as having a pre-defined role as helper T cells within the immune system . For example, when an antigen - presenting cell expresses an antigen on MHC class II, a CD4 cell will aid those cells through a combination of cell to cell interactions (e.g. CD40 (protein) and CD40L) and through cytokines . </P>

What is the function of t helper cells