USMLE Step 1

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Immunology

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B Cells and T Cells

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T Cells - Overview

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Summary

T-cells are lymphocytes which are part of the adaptive immune system. T-cells mediate cellular immunity, usually through direct contact with other cells. When T-cells overreact to self-antigens or other harmless antigens, undesired T-cell responses cause Type 4 hypersensitivity reactions. 

Key Points

  • T cells
    • Lymphocytes of the adaptive immune system
      • Must be activated against a specific antigen
      • Most (80%) circulating lymphocytes are T-cells 
    • Types include Helper CD4+ T-cells and Killer CD8+ T-cells
    • Mediate cell-mediated immune response 
      • Achieves its effects by direct cell-to-cell contact
    • Undesired T cell activity results in Type 4 Hypersensitivity Reactions
      • Also called delayed or cell-mediated hypersensitivity