Medicine & USMLE


  1. Innate Immunity
  2. Adaptive Immunity
  3. Macrophages
  4. Neutrophils
  5. Dendritic Cells
  6. Mast Cells
  7. Eosinophils
  8. Basophils
  9. Natural Killer Cells
  10. Antigens
  11. MHC I and II
  12. Antibodies
  13. B Lymphocytes Overview
  14. B Cell Stages
  15. B-Cell Activation
  16. Plasma B-Cells
  17. Memory B-Cells
  18. T-Lymphocytes Overview
  19. T-Cell Stages
  20. Types of Activated T-Cells


Macrophages, also known as monocytes while circulating in blood, are a major type of white blood cell in the innate immune system. Macrophages primarily work to fight infection by acting as phagocytes, or cells that eat things like foreign pathogens to break them down. Macrophages can also take little bits of what they eat, and present them on their cell surface to T-cells, in what is formally known as antigen presentation. This antigen presentation allows the activation of the adaptive immune system against the threat, thereby bridging the innate and adaptive immune systems. Lastly, macrophages also secrete cytokines that signal to and attract other immune cells to help fight infection.

Key Points

  • Macrophages
    • Innate immune cell
    • Phagocyte
      • Foreign pathogens are put into phagosomes, which fuse with lysosomes to break down the pathogen
    • Presents antigens
      • Bridge between innate and adaptive immune response
      • Phagocytosed and digested antigens are presented on cell surface for recognition by T-cells
      • Macrophages can present antigens, but Dendritic Cells are more likely to do so
    • Coordinates (innate) immune response via release of cytokines
      • Cytokines are secreted proteins that signal to other immune cells
      • Macrophages release cytokines to increase or decrease inflammation, and recruit other immune cells to affected sites