Immunity — Host Defence

Immune System: complex network of cells and proteins that defends the body against infection.

Physical barriers

• Skin
• Cilia
  • Respiratory tract
• Acidic environment
  • Stomach

Innate immunity

Recognize generic pathogenic motifs and directly attack or opsonize and phagocytose

• Defensins – perforate pathogenic membranes
• Compliment cascade – opsonize and signal for phagocytosis and directly lyse via membrane attack complex
• Innate immune cells
  • Neutrophils
    • Degranulate cytotoxic contents and cause oxidative damage to pathogens
  • Macrophages
    • Phagocytose and kill opsonized pathogens
  • Dendritic cells
    • Phagocytose and present antigen to cells of acquired immune system
  • Natural Killer cells
    • Recognize abnormal cell surface expression (e.g. lack of MHC, abnormal MHC proteins) due to intracellular pathogens (e.g. virus) and directly kill infected cells

Acquired immunity

• Pathogenic antigens presented to B cells and T cells via APCs such as dendritic cells
• Show: Specificity, Memory, Diversity, Immunological tolerance
• B cells (20%, Humoral Immune reponse)
  • Produce antibody against antigen
  • Antibodies opsonize and target for T cell killing, phagocytosis and compliment cascade
  • B cells become memory B cells and plasma cells, primed for explosive antibody production on subsequent exposure to the same antigen
• T cells (80%, Cell-mediated immune reponse)
  • Major subtypes into CD8 (Killer) T cells and CD4 (Helper) T cells
  • CD8 (Cytotoxic/Killer) T cells recognize specific antigen presented to them via TCR and lyse pathogens and infected cells
  • CD4 T (Helper) cells produce cytokines for adjacent B cells that recognize the same antigen, allowing B cell differenciation and antibody production
  • CD28 (Regulatory) T cells dampen the immune reaction, preventing or limiting autoimmune damage.
  • Memory T Cell: Reside in lymphoid tissue and induces more rapid and powerful immune response with 2nd exposure to Ag

Sakurai 2016

Answer by Sakurai