Chronic inflammation. Etiology, types, morphology
Chronic inflammation is inflammation over a period of weeks, months or years. Typically considered longer than 6 months. The main characteristics of chronic inflammation are:
- Infiltration with mononuclear cells (macrophages, lymphocytes, plasma cells)
- Destruction of tissue
- Angiogenesis and fibrosis in attempted tissue repair
The three main causes of chronic inflammation are:
- Persistent infection
- Immune-mediated inflammatory diseases
- Toxic agents
Chronic inflammation can be the end result of acute inflammation, or may arise from the following:
- Persistent infections (e.g. Mycobacterium tuberculosis)
- Immune-mediated inflammatory diseases
- Prolonged exposure to toxic agents
Examples: typical diseases for each cause:
- Persistent infection: tuberculosis, syphilis, chronic viral hepatitis
- Immune-mediated: rheumatoid arthritis, Crohn disease, asthma
- Toxic agents: inhaled silica (silicosis); cholesterol in the arterial wall (atherosclerosis)
Cells and mediators
- Macrophages
- Dominant cells of chronic inflammation
- Known as Monocytes in the blood, then move to the tissue as macrophages, activated by diverse stimuli, either via classical or alternative activation
- Classical - induction by endotoxin and T cell derived signals (IFN-gamma), have microbicidal actions and trigger inflammation
- Alternative - induction by cytokines other than IFN-gamma, result in Tissue repair, fibrosis and anti-inflammatory effects
- Lymphocytes
- Activation of T and B cells
- B-lymphocytes - plasma cells produce antibodies
- T-cells - T Helpers, suppressors and Killers
The interactions between these two classes of cells are shown below.