Lecture 3 - Innate Immunity (the cells)

• Inflammation

  1. Occurs as a result of infection or tissue damage

  2. Characteristics include heat, redness, swelling, and pain

  3. Plays roles in delivering effector cells and molecules, blood clotting, and enhancing tissue repair

  4. Changes in blood vessels include increased diameter, adhesion molecules, and vascular permeability

  5. Attachment of leukocytes to the endothelium followed by migration to infected tissue is called extravasation
• Phagocytosis

  1. Occurs in phagocytes such as neutrophils, macrophages, and dendritic cells

  2. Involves targeting pathogens coated in opsonins

  3. Steps include attachment, engulfment, phagosome formation, recruitment of lysosomes, fusion, and microbe killing

  4. Requires membrane rearrangement and opsonins

  5. Microbe killing due to reactive oxygen species, defensins, hydrolytic enzymes, and low pH in phagolysosome

  6. Neutrophils release azurophilic and specific granules, as well as neutrophil extracellular traps
• Neutrophils are the first responders to infection or injury, being recruited to the site within minutes
• Neutrophils are short-lived, quick-acting, and motile, making them effective at combating pathogens
• Neutrophils are a type of white blood cell, making up 40% to 75% of the total white blood cell count
• Neutrophils kill pathogens through phagocytosis, degranulation, and the release of neutrophil extracellular traps
• Neutrophils are part of the polymorphonuclear family, along with basophils and eosinophils
• Innate immunity

  • The body's first line of defence against pathogens and is not specific to particular pathogens
  • Inflammation is crucial in innate immunity, as it helps to clear infections and repair tissue damage
  • Phagocytosis is a key mechanism of innate immunity
  • Neutrophils, macrophages, dendritic cells, and natural killer cells are important components of the innate immune system
  • The innate immune response is immediate, fast, and does not create a long-term memory of the pathogen
  • Innate immune cells are derived from both the myeloid and lymphoid lineages, each with distinct functions
  • Natural killer cells play a crucial role in recognising and killing infected or abnormal cells
• Dendritic cells


  • Professional antigen-presenting cells
  • Process microbial proteins to small peptides for recognition by T lymphocytes
  • Up-regulate expression of surface proteins and cytokines for optimal antigen presentation
  • Secrete type I interferons, migrate to lymph nodes, and link the innate with the adaptive arms of the immune system
• Neutrophils


  • Kill pathogens through phagocytosis, degranulation, and the release of neutrophil extracellular traps
  • Engulf pathogens and destroy them within the phagolysosome
  • Release azurophilic and specific granules containing proteases, defensins, ROS, and antimicrobial proteins to trap and kill bacteria
• Natural killer cells (NK cells)

  • Terminally differentiated cells of the lymphoid lineage
  • Contain cytotoxic proteins like granzyme and perforin to kill infected cells
  • Secrete interferon-gamma, become activated in response to type I interferons, and control cytotoxic activity through inhibitory and activating receptors
  • Responsible for the clearance of "missing self" cells and can perform Antibody-Dependent Cellular Cytotoxicity (ADCC)
• Macrophage activation

  Direct interaction with pathogens or factors secreted by T helper lymphocytes such as IFN gamma and IL4
• Macrophages

  • Belong to the myeloid lineage and are a mature form of monocytes
  • Reside in tissues such as microglia in the brain, Kupffer cells in the liver, and osteoclasts in bone
  • Phagocytose microbes, secrete cytokines, present antigens to T lymphocytes, and contribute to tissue repair and the formation of new blood vessels
• Innate lymphoid cells (ILCs)

  • Translate signal cytokines into effector cytokines without expressing an antigen-specific receptor
  • Mirror the phenotypes and functions of T cells, respond to signals from infected or injured tissues, produce cytokines, but do not respond directly to antigens or undergo clonal selection upon stimulation