A T cell, characterized by having CD4 protein antigens on its surface, that activates other cells of the immune system. CD4 T cells differentiate into several subsets; the two most significant are called T_H1 and T_H2. The T_H1 cells have an important role in controlling intracellular viral and bacterial infections. Pathogens can grow within intracellular vesicles of macrophages unless the macrophage is stimulated by binding of a T_H1 cell. A T_H1 cell binds to an antigen-presenting cell (i.e. a macrophage) that is displaying the specific antigen for the T-cell receptor. This prompts the T cell to secrete cytokines such as γ‎-interferon (IFN-γ‎), which activate the macrophage, causing intracellular vesicles to fuse with lysosomes inside the macrophage, enabling the lysosomal contents to destroy the bacteria. Exchange of cytokines between the antigen-presenting cell and the T cell causes the T cell to proliferate, producing a clone of T cells all with the same antigenic specificity. T_H2 cells have a vital role in helping to activate B cells and initiate their proliferation and differentiation into antibody-producing plasma cells. They also coordinate immune responses to large extracellular pathogens. B cell activation occurs when an antigen-specific ‘armed’ T cell recognizes its antigen presented on the surface of a B cell in association with MHC class II protein. The T_H2 cell becomes attached to the B cell and synthesizes various cell-bound and secreted molecules, notably the CD40 ligand (CD40L, or CD154) and interleukin 4 (IL-4). CD40L binds to the CD40 molecule on the B-cell surface, triggering a suite of responses by the B cell. Other subsets of CD4 T cells include regulatory T cells (T_regs), which are vital in maintaining homeostasis of the immune system. This involves suppressing immune responses that are no longer needed and ensuring that the system continues to distinguish between self and nonself material.