of an infected target cell by localized exocytosis at an immunological synapse. The high concentration of ca2+ in the extracellular fluid causes the perforin to assemble into transmembrane channels in the target-cell plasma membrane, allowing the granzymes to enter the target-cell cytosol. The granzymes cleave and activate procaspases to initiate a caspase cascade, leading to apoptosis (see Figure 18–3). A single cytotoxic cell can kill multiple target cells in sequence. It remains a mystery why the released perforins do not form pores in the plasma membrane of the killer cell itself target cell (Movie 24.12 and Movie 24.13).
effector helper T cells help Activate Other cells of the Innate and Adaptive Immune Systems
In contrast to TC cells, helper T cells (TH cells) are crucial for defense against both extracellular and intracellular pathogens, and they express CD4 rather than CD8 co-receptors and recognize foreign peptides bound to class II rather than class I MHC proteins. Once naïve TH cells are induced on activated dendritic cells to become effector cells, they can help activate other cells: they help activate B cells to become antibody-secreting cells and later to undergo Ig class switching and somatic hypermutation; they help activate macrophages to destroy any intracellular pathogens multiplying within the macrophage’s phagosomes; they help induce naïve TC cells to become effector cells that can kill infected target cells; and they stimulate the activated dendritic cell that activated them to maintain the dendritic cell in an activated state. In each case, the effector TH cell recognizes the same complex of foreign peptide and class II MHC protein on the target-cell surface that it initially recognized on the activated dendritic cell. As discussed later, the TH cell stimulates the target cell both by secreting a variety of cytokines and by displaying co-stimulatory proteins on its surface.
Naïve helper T cells can Differentiate Into Different Types of effector T cells
When activated by binding to a foreign peptide bound to a class II MHC protein on an activated dendritic cell, a naïve TH cell can differentiate into several distinct types of effector T cells, depending on the nature of the pathogen and the cytokines they encounter. These cells include four subtypes of helper cells—TH1, TH2, TFH, and TH17 cells—and regulatory (suppressor) T cells. Figure 24–44 summarizes both the cytokines that induce these effector T cells and some of the cytokines the effector cells secrete, as well as the master transcription regulators that control the effector cell’s development.
Naïve TH cells activated by dendritic cells secreting the cytokine interleukin-12 (IL12) develop into TH1 cells. These effector cells produce interferon‑γ (IFNγ),
Figure 24–44 Differentiation of naïve helper T cells into different types of effector helper cells or regulatory  T cells in a peripheral lymphoid organ.
The cytokines produced by the activating dendritic cell (and by other cells in the environment) mainly determine which type of effector T cell develops, as indicated. Some of the main cytokines produced by each type of effector cell are also shown, and the master transcription regulator for each subset is indicated in the nucleus. There is increasing evidence that some of the effector cells are plastic and can change the cytokines they produce in response to changes in their environment (not shown).