Cell Mediated Immunity

Last updated on May 13th, 2021

CELL-MEDIATED IMMUNITY: Antibody-independent immunity, mediated by TH1 cells.
Examples: Type-IV Hypersensitivity responses against Mycobacterium tuberculosis and other intracellular pathogens.

  • MACROPHAGE ACTIVATION: IFN-gamma, secreted by TH1 cells, activates macrophages.
    • Increase expression of MHC-II molecules in macrophages, to enhance their roles as APC’s.
    • Increase expression of Fc-receptors and CR3-receptors, to enhance opsonization.
    • Increase oxidative-burst ——> reactive O2-intermediates and reactive N2 intermediates (such as NO).
    • Increase synthesis of eicosanoids: PGD2, PGI2, TXA2, LTB4
  • CYTOTOXIC LYMPHOCYTE (CTL) ACTIVATION: IL-2 converts CTL-Precursors ——> Active CTL
    • IL-2 is secreted by TH1 cells.
    • Antigen activation of a CD8 cell causes it to up-regulate its expression of IL-2 Receptors. Subsequent IL-2 activation (by TH1 cells) causes CTL proliferation.
    • After antigen activation, both CTL’s and TH-Cells are dependent on IL-2 for their proliferation.
    • TERMINATION: After antigen clearance, the levels of IL-2 decline, and CTL’s die by apoptosis. This ensures that the immune response is limited and helps to prevent excessive tissue damage.
    • CONJUGATE FORMATION: CTL binds to MHC-I of the target cell (which is expressing antigen and is therefore targeted for destruction).
      • TCR-CD3 complex on CD8-Cell binds to HLA-I of target cell
      • Additional binding by LFA-1 of CD8 cell with ICAM-1 of target cell.
    • PERFORIN MONOMERS: After conjugate formation, the CD8 cell releases perforin monomers from storage granules.
      • These monomers polymerize within the membrane to form perforin pores ——> target-cell membrane lysis.
    • DEATH BY APOPTOSIS: Some evidence exists that the CTL cell sends a signal to target cell which causes the target-cell to die by apoptosis.
      • TNF-beta has been a signal that is implicated in this apoptosis signal.
  • NATURAL KILLER (NK) CELLS: Non-specific (natural) killing of tumors and some virally infected cells.
    • They are believed to derive from null cells. Strange cell lineage.
    • KILLING METHOD: Similar to CTL’s.
      • They degranulate perforin-like monomers to cause target-cell lysis.
      • They kill cells by apoptosis (mediated by TNF-alpha)
      • They do not express CD8, CD3 or any other CD lymphocyte markers on their membranes.
      • They have no immunologic memory.
  • ANTIBODY-DEPENDENT CELLULAR CYTOTOXICITY (ADCC): Many cytotoxic cells express Fc-receptors on their membranes. They can then kill an antibody(IgG)-coated cell by a non-specific process.
    • Cells involved: Most natural immunity cells have Fc-Receptors and can thus partake in ADCC. Neutrophils, Eosinophils, Macrophages, NK-Cells, Monocytes
    • Cells coated with IgM are not subject to ADCC because IgM has no available Fc region!
      Primarily IgG is the antibody responsible for ADCC.

About Acharya Tankeshwar 474 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.