Makindo Medical Notes

Overview of Phagocytes

Phagocytes are a type of immune cell that engulfs and digests foreign particles, bacteria, and dead or dying cells. This process is known as phagocytosis. Phagocytes play a crucial role in the immune system by helping to protect the body against infections and maintaining tissue homeostasis.

Types of Phagocytes

• Neutrophils:
  • Most abundant type of white blood cell.
  • First responders to sites of infection or injury.
  • Engulf and destroy bacteria and fungi.
  • Short-lived and form pus when they die.
• Macrophages:
  • Derived from monocytes that have migrated into tissues.
  • Engulf and digest pathogens, dead cells, and cellular debris.
  • Present antigens to T cells to initiate adaptive immune responses.
  • Longer-lived than neutrophils and can reside in tissues for extended periods.
• Dendritic Cells:
  • Act as antigen-presenting cells that capture antigens and present them to T cells.
  • Found in tissues that are in contact with the external environment, such as the skin and mucous membranes.
  • Key players in initiating adaptive immune responses.
• Eosinophils:
  • Primarily involved in combating parasitic infections and allergic reactions.
  • Engulf and destroy parasites and modulate inflammatory responses.
• Basophils:
  • Play a role in allergic reactions and parasitic infections.
  • Release histamine and other inflammatory mediators.
  • Contribute to the inflammatory response but are less involved in phagocytosis compared to other phagocytes.

Mechanism of Phagocytosis

• Chemotaxis:
  • Phagocytes are attracted to the site of infection or injury by chemical signals, such as cytokines and chemokines.
• Adherence:
  • Phagocytes recognize and bind to pathogens or particles through receptors on their surface.
  • Opsonization enhances adherence, where pathogens are coated with antibodies or complement proteins.
• Engulfment:
  • The phagocyte extends its membrane around the pathogen or particle, forming a phagosome.
• Digestion:
  • The phagosome fuses with a lysosome to form a phagolysosome.
  • Digestive enzymes and reactive oxygen species within the phagolysosome destroy the pathogen or particle.
• Exocytosis:
  • Waste materials and undigested particles are expelled from the phagocyte.

Role in Immune Response

• Innate Immunity:
  • Phagocytes are a key component of the innate immune system, providing an immediate response to infection.
  • Recognize and respond to a wide range of pathogens through pattern recognition receptors (PRRs).
• Adaptive Immunity:
  • Phagocytes, particularly macrophages and dendritic cells, present antigens to T cells, bridging innate and adaptive immunity.
  • Help to activate and shape the adaptive immune response.
• Inflammation:
  • Phagocytes release cytokines and other inflammatory mediators that recruit additional immune cells to the site of infection or injury.
  • Play a central role in the initiation and resolution of inflammation.

Clinical Significance

• Infections:
  • Phagocytes are crucial in defending against bacterial, fungal, and parasitic infections.
  • Deficiencies in phagocyte function can lead to increased susceptibility to infections.
• Autoimmune Diseases:
  • Phagocytes can contribute to tissue damage in autoimmune diseases by attacking the body's own cells.
• Inflammatory Disorders:
  • Chronic activation of phagocytes can lead to persistent inflammation and tissue damage in conditions such as rheumatoid arthritis and inflammatory bowel disease.
• Cancer:
  • Phagocytes can influence tumour progression by either promoting anti-tumour immunity or contributing to tumour growth and metastasis.

Summary

Phagocytes are essential immune cells that protect the body by engulfing and digesting foreign particles, pathogens, and dead or dying cells. They include neutrophils, macrophages, dendritic cells, eosinophils, and basophils. Phagocytosis involves chemotaxis, adherence, engulfment, digestion, and exocytosis. Phagocytes play a crucial role in both innate and adaptive immunity, inflammation, and maintaining tissue homeostasis. Understanding their function and clinical significance is important for diagnosing and treating various infectious, autoimmune, inflammatory, and cancerous conditions.