Adaptive Immune Response

Overview

• Also called acquired or specific immunity
• Tailored to target specific pathogens
• Two branches: cellular (cell-mediated) immunity and humoral (antibody-mediated) immunity

Cellular Immunity

T-Lymphocytes

• Major players in cellular immunity
• Develop in the thymus
• Billions of variations, each with a unique T-cell receptor (TCR)
• Subpopulations:
  • Helper T-cells: Have CD4 receptor
    • Assist in activation of other immune cells
  • Cytotoxic T-cells: Have CD8 receptor
    • Directly kill infected or cancerous cells
• Learn to not react to the body's own antigens; self-reactive T-cells are eliminated
• Mature T-cells migrate to lymph nodes and lymphoid tissues
• Activation occurs when T-cells match a pathogen's antigen presented via a host molecule (MHC)

Major Histocompatibility Complex (MHC)

• MHC Class I:
  • Expressed by all nucleated cells
  • Displays peptides on cell surface
  • Activates cytotoxic T-cells (CD8 receptor)
• MHC Class II:
  • Found on professional antigen-presenting cells (APCs), especially dendritic cells
  • Displays antigens to T-helper cells (CD4 receptor)
  • Dendritic cells migrate to lymph nodes to present antigens

Activation and Clonal Expansion

• T-cell activation requires a second binding verification step
• Activated T-cells undergo clonal expansion (mitosis)
• Produce identical clones:
  • Effector T-cells
    • Helper T-cells: Produce interleukins
    • Cytotoxic T-cells: Kill infected or cancerous cells
  • Memory T-cells: Provide faster response upon reexposure
• Effector cells migrate to infection site, Memory cells remain in lymph nodes or circulate

Regulatory T-cells

• Downregulate immune response post-infection
• Differentiate between self and non-self antigens
• Prevent autoimmune diseases

Summary

• Helper T-cells central to adaptive immunity
• Cytotoxic T-cells main actors in cellular immunity
• Memory cells ensure quicker response to future infections
• Regulatory T-cells maintain immune balance