Chronic inflammation is characterized by a persistent inflammatory response, marked by the presence of mononuclear cells (macrophages, lymphocytes, and plasma cells), tissue destruction, and subsequent repair processes, including fibrosis and angiogenesis.

1. Macrophages
   • Origin and Function
     • Derived from blood monocytes that migrate into tissues and differentiate into tissue macrophages.
     • Have a long life span in connective tissue (≈ 60–120 days).
     • Function as phagocytes, antigen-presenting cells, and cytokine producers.
   • Tissue-Specific Macrophages
     • Connective tissue: Histiocytes
     • Lung: Alveolar macrophages
     • Liver: Kupffer cells
     • Bone: Osteoclasts
     • Brain: Microglia
   • Key Functions
     • Phagocytose necrotic cells and pathogens.
     • Secrete monokines (e.g., IL-1, TNF-α) to recruit lymphocytes.
     • It can transform into epithelioid cells in granulomatous inflammation.
2. Lymphocytes
   • Types
     • B cells: differentiate into plasma cells that produce antibodies.
     • T cells: Regulate and mediate immune responses (helper, cytotoxic, regulatory)
   • Lymphotaxin serves as a lymphocyte chemokine, attracting lymphocytes to inflamed tissues.
   • Lymphocytes interact closely with macrophages—reciprocal activation amplifies chronic inflammation.
3. Eosinophils
   • Morphology
     • Bilobed nucleus with large, red (eosinophilic) granules.
   • Functions
     • Crucial in parasitic infections and IgE-mediated allergic reactions.
     • Granules contain major basic protein (MBP) → toxic to parasites.
     • Recruited by eotaxin (the eosinophil chemokine).
     • Respond to C5a, MCP-1, MIP-1α, PDGF, and TGF-β.
4. Basophils and Mast Cells
   • Basophils
     • Location: Circulating in the blood.
     • Granule content: Histamine, heparin
     • Role: Mediate allergic and anaphylactic reactions
     • ReceptorL: High-affinity IgE receptors
   • Mast Cells
     • Location: Tissue-resident (esp. skin, lung).
     • Granule content: Histamine, heparin
     • Role: Mediate allergic and anaphylactic reactions
     • ReceptorL: High-affinity IgE receptors

Both release histamine during IgE-mediated reactions. Basophils circulate in the bloodstream, while mast cells remain fixed in tissues.

Summary Table: Key Cells in Chronic Inflammation

Learning Objective

Understand the key cellular participants in chronic inflammation, including their origins, mediators, and roles in immune response, tissue injury, and repair. Recognize how macrophages, lymphocytes, eosinophils, and basophils interact to sustain chronic inflammation and mediate immune-related pathology.

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