Microbiology · MU Sofia

35. Cell-mediated immunity

Explains cell-mediated immunity as the T-cell, macrophage and NK cell response that works without antibodies, then covers the roles of CD4+ TH1 and CD8+ cytotoxic T cells, macrophages in granuloma formation, and NK cells. It closes with MHC structure, the two classes and their antigen presentation, and MHC restriction, including how B cells act as antigen-presenting cells. Several diagrams are included.

Inside: 8 sections · 4 labelled pictures · 14 flashcards · 6 min

Topic 35 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 8 sections, about 808 words (6 min of reading), with 14 flashcards.

In this note

  1. Cell-mediated immunity. Role of the major histocompatibility complex in the immune response.
  2. Cell-Mediated Immunity (CMI)
  3. Role of T-cells in CMI
  4. Role of Macrophages in CMI
  5. Role of Natural Killer (NK) Cells
  6. Major Histocompatibility Complex (MHC)
  7. Classes of MHC Molecules
  8. MHC Restriction and Antigen Presentation

Cell-Mediated Immunity (CMI)

  • Cell-mediated immunity (CMI) is an immune response that does not rely on antibodies.
  • Instead, CMI is an immune response that involves the actions of phagocytes, T-lymphocytes, and the release of specific cytokines in response to an antigen. It is considered the other half of the adaptive immune system, alongside humoral immunity.
  • The primary immune cells involved in CMI are T-cells, especially CD4+ T helper cells (specifically TH1 cells) and CD8+ cytotoxic T-cells (Tc cells), and macrophages. NK cells are also involved.
  • CMI is primarily directed against microbes that survive phagocytosis and those that infect non-phagocytic cells, such as bacteria, parasites, and viruses that replicate inside cells where they cannot be destroyed by antibodies.
  • It is especially important in defending against intracellular pathogens, tumour cells, and graft rejection.

Role of T-cells in CMI

T-lymphocytes are involved in CMI. They arise from bone marrow stem cells, migrate to the thymus for maturation, and then move to peripheral lymphoid tissues to encounter antigens.

  • CD4+ T helper cells (TH cells):
  • Activation requires recognition of antigen in association with class II MHC molecules.
  • Once activated, they secrete cytokines that activate and regulate other immune cells like B cells, cytotoxic T cells, and macrophages.
  • The TH1 subclass is particularly involved in defence against intracellular pathogens growing in phagosomes (e.g., mycobacteria).
  • TH1 cells secrete IFNγ, which activates macrophages to destroy ingested microbes. They also stimulate the production of new macrophages in the bone marrow and recruit fresh macrophages to infection sites.
  • CD8+ cytotoxic T cells (TC cells):
  • Activation involves recognition of antigen in association with class I MHC molecules.
  • They act against cells infected with viruses. Viral antigens on infected cells are recognized by antigen receptors (TCRs) on the cytotoxic T cells.
  • They kill or lyse infected or abnormal cells by releasing cytotoxic proteins (like perforin) that initiate apoptosis of the target cell.
  • CD8+ T cells also have non-lytic functions mediated by cytokine production, mainly IFNγ.

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