1. Adaptive Defenses T Lymphocytes T lymphocytes constitute the "cellular" arm of acquired/specific immunity T lymphocytes play a central role in controlling the acquired immune response Serve as crucial effector cells through antigen specific cytotoxic activity and the production of soluble mediators called lymphokines (the cytokines produced by lymphocytes)

2. Adaptive Defenses T Lymphocytes There are two major subsets of T lymphocytes that differ in effector function, MHC restriction and accessory molecule usage helper T cell (CD4+Tcells) cytotoxic T cell (CD8+Tcells)

3. Adaptive Defenses Lymphocyte Functions T cells secrete lymphokines ( cytokines) help activate T cells cause T cell proliferation activate cytotoxic T cells stimulate leukocyte production stimulate B cells to mature activate macrophages secrete toxins that kill cells secrete growth-inhibiting factors secrete interferon cellular immune response

4. Adaptive Defenses cytokines (Lymphokines) Cytokines are basically polypeptide hormones The term is often used to denote those molecules which are the products of cells of the immune system or which act upon such cells In common with other hormones, cytokines exert their effects by binding to specific cell- surface receptors which signal to their target cells

5. Adaptive Defenses cytokines (Lymphokines) Cytokines act at very low concentrations They are short-lived and may act locally, either on other cell types (paracrine) or on the same cell type (autocrine), or systemically (endocrine)

6. Adaptive Defenses Antigen presentation T cells must see antigen in the form of peptides bound to self MHC molecules Class I MHC is adapted for mainly antigenes produced inside the cells Class II MHC is specially adapted to present extracellular antigens

7. Adaptive Defenses Antigen presentation Three sets of cells can present antigens to T lymphocytes, which are named Antigen Presenting Cells (APCs) APCs are Dendritic cells, activated Macrophages or B cells, which have both clas I and class II MHC

8. Adaptive Defenses Antigen presentation Most real antigen presentation situations require the binding to a receptor which triggers endocytosis This receptor can be for complement or the Fc of IgG or it can be an innate, carbohydrate receptor on macrophages/dendritic cells. In normal situations the critical antigen presenting cell (APC) is the dendritic cell

9. Adaptive Defenses Antigen presentation The role of dendritic cells is to act as antigen sensors They recognize antigen via their innate (carbohydrate or complement) receptors These receptors can trigger activation (upregulation of antigen presenting and accessory functions) and migration from the peripheral tissue to the lymph node via the afferent lymphatics

10. Adaptive Defenses Antigen presentation Within the lymph node the antigen presenting DC can interact with the many T cells constantly trafficking through. Interact with MHC/peptide – TCR The principal one which concerns us is the association of the CD28 molecule on the T cell with either of 2 ligands B7.1 (CD80) and B7.2(CD86) molecule on the dendritic cell.

11. Adaptive Defenses Antigen presentation These molecules are termed accessory molecules and we know that the CD28 molecule delivers an essential second signal to the T cell without which the T cell does not become activated (indeed it becomes unresponsive)

12. CYTOKINES, T CELL SUBSETS CD4+ helper T cells are capable of differentiating from an initial common state (T H ) into 2 apparently distinct types called T H 1 and T H 2. These subtypes differ in their cytokine secretion

13. CYTOKINES, T CELL SUBSETS

14. The commitment of TH0 cells to become TH1 or TH2 is influenced by cytokines secreted by the 2 subtypes themselves and by macrophages, NK cells and mast cells as shown in the following diagram

15. CYTOKINES, T CELL SUBSETS

16. CYTOKINES, T CELL SUBSETS AND Ig ISOTYPE REGULATION The outcome of this differentiation switch is to activate 2 different pathways of immunity which are associated with different antibody isotypes The TH1 pathway is essentially cell mediated immunity, with the activation of macrophages, NK cells, cytotoxic T cells and a prolonged inflammatory response The cytokines secreted by TH1 cells also boost production of IgG2a antibody production in mice

17. APC IFN-γ CD4 T cell Th1 CD8 T cell IFN-γ Cytotoxic activity Macrophage Amastigotes Pathogens Promastigotes Adaptive Defenses Cellular Immunity