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 two subsets, called TH1 and TH2. The TH1 cells have an important role in controlling intracellular bacterial infections, such as the mycobacteria responsible for tuberculosis and leprosy. These can grow within intracellular vesicles of macrophages, unless the macrophage is stimulated by binding of effector TH1 cells via the T-cell receptor, prompting the T cell to secrete cytokines such as γ-interferon (IFN-γ). These cause the intracellular vesicles to fuse with lysosomes inside the macrophage, enabling the lysosomal contents to destroy the bacteria. TH2 cells have a vital role in helping to activate B cells and initiate their proliferation and differentiation into antibody-producing plasma cells. This 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 TH2 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.
Subjects: Biological Sciences.