T Cells- 70–75% circulating lymphocytes
- Arise in bone marrow, mature in thymus, and divide into helper T cells (CD4) and cytotoxic T cells (CD8); recognize fragments of Ag/peptide bound to MHC molecule on cell surface and play major role in immune regulation via secretion of cytokines or direct cell lysis
- Need two signals for activation: TCR/MHC and CD28/B7 binding
- Helper T cells further subdivided into TH1 and TH2 cells
- TH1 cells
- Mediate delayed-type contact hypersensitivity reactions, ↑ cell-mediated immunity, activate macrophages, produce IL-2, ↑ IgG production, ↓ IgE production, downregulate TH2 response via IFNγ
- Optimal immunity against viruses and intracellular bacteria
- TH1 cytokines: IL-2, IL-12, IFN γ (latter main macrophage activating cytokine), and TNFα
- Diseases associated with TH1 cytokine profile: allergic contact dermatitis, tuberculoid leprosy, cutaneous leishmaniasis, psoriasis (latter TH17)
- TH2 cells
- Upregulate humoral immunity by producing IL-4 and IL-5, activate eosinophils, induce switching to IgG4 and IgE, and suppress macrophage activity, downregulates TH1 response via IL-10
- Optimal immunity against viruses, extracellular bacteria and parasites
- TH2 pattern: IL-4, IL-5, IL-6, and IL-10
- Diseases with TH2 profile: atopic dermatitis, lepromatous leprosy, disseminated leishmaniasis, Sezary syndrome, and parasitic infections
- Cytotoxic (CD8) cells
- Results in lysis of cells (either foreign or host cells containing foreign proteins): CD8 binds MHC I during Ag presentation → CD8 cell releases perforin, granzymes, and granulysin → apoptosis of target cell; may also undergo apoptosis via FAS/FAS ligand
| | | | | | | Table 1-13 Surface Molecules of T Cells | | Surface Molecule on T Cell | | Function | | | CD2 | | Binds LFA3 on Ag presenting cell (APC) | | | CD3 | | Present on all T cells | | | CD4 | | Seen in helper T cells, binds MHC II | | | CD8 | | Seen in cytotoxic T cells, binds MHC I | | | CD28 | | Binds B7 on APC | | | LFA1 (leukocyte function antigen) | | Binds ICAM1 on APC (promotes adhesion) | | | CCR5 | | Co-receptor used for HIV entry | | | | | |
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