Activation and differentiation of T lymphocytes. Dendritic cells (DCs) take up antigen delivered either by an infectious pathogen or by a vaccine (stage 1). DCs are activated by conserved components of pathogens binding to pattern recognition receptors (PRRs) that induce the expression of co-stimulatory molecules and the release of inflammatory cytokines. Vaccines that do not contain intrinsic adjuvants are delivered with added adjuvants, such as aluminum salts, that also activate inflammatory pathways. DCs degrade the antigen into peptides that are returned to the cell surface on MHC molecules and presented to CD8 and CD4+ T cells - antigen being presented to CD8+ T cells by MHC class I molecules, and to CD4+ T cells on MHC class II molecules (stage 2). T cells also require signals provided by the co-stimulatory molecules and inflammatory cytokines to be fully activated. Activation results in cell proliferation (stage 3) and effector cell differentiation (stage 4). CD4+ T cells can make cytokines that activate innate immune cells (such as macrophages and neutrophils) to kill pathogens. Activated CD4+ T cells can also provide help to B cells, expressing the cell-surface and soluble mediators required for the production of high-affinity class switched antibody. Effector CD8+ T cells can kill infected cells by releasing cytotoxic granules or can activate other cell types by the release of inflammatory cytokines.