Inflammation is the response of tissue to injury and is a series of processes initiated to limit damage to tissue. Chronic inflammation arises as a response to injury with associated fibrosis and takes place over a longer period of time than acute inflammation. It is a far less stereotyped process than acute inflammation and has an overlap with host immunity.
This article shall consider how chronic inflammation can arise, the cell types involved and clinical examples.
How Can It Arise?
There are a number of situations in which chronic inflammation may arise:
- It may ‘take over’ from acute inflammation if the damage cannot be resolved in a few days
- It may arise de novo, e.g. in autoimmune conditions such as rheumatoid arthritis
- It may develop alongside acute inflammation in severe persistent/repeated irritation
The microscopic appearance of chronic inflammation is much more varied than that of acute inflammation. It is generally described in terms of the cells that are present:
- Macrophages – present in acute and chronic inflammation, important for phagocytosis, antigen presentation and cytokine synthesis
- Lymphocytes – immunological functions, B cells differentiate to produce antibodies and T cells have cytotoxic functions
- Plasma cells – differentiated antibody producing B lymphocytes, the presence of these usually indicates the inflammation has been present for a considerable amount of time
- Eosinophils – often found in allergic reactions and parasitic infections
- Fibroblasts/Myofibroblasts – recruited by macrophages, produce collagen to assist in healing and repair
Generally the morphology of chronic inflammation is fairly non-specific however the proportions of each cell type will vary depending on the condition. For example, rheumatoid arthritis presents with mainly plasma cells, whereas in chronic gastritis lymphocytes are typically most abundant.
Giant cells are multi-nucleated cells that are made by the fusion of multiple macrophages. They form as a result of frustrated phagocytosis – when a phagocyte fails to engulf its target. There are several types recognised in different conditions:
- Langhans Giant Cell in tuberculosis
- Foreign Body Type Giant Cell
- Touton Giant Cell in fat necrosis
Granulomas are a collection of epithelioid histiocytes (macrophages) that may form in chronic inflammation. They may also have associated lymphocytes or an area of central necrosis.
They arise as a result of persistent, low-grade antigenic stimulation or hypersensitivity and the immune system attempts to ‘wall off’ a substance that it is unable to eliminate. Some examples of diseases which feature granulomatous inflammation include Tuberculosis, leprosy and Crohn’s disease.
Effects of Chronic Inflammation
Chronic inflammation can have a number of complications depending on the area and disease process it is associated with. These include:
- Fibrosis, e.g. chronic cholecystitis can lead to fibrosis of the gall bladder wall
- Impaired function, e.g. inflammatory bowel disease
- Atrophy, e.g. atrophy of gastric mucosa in gastritis
- Stimulation of immune response, e.g. local and systemic immune effects of rheumatoid arthritis
Clinical Relevance – Pulmonary Tuberculosis
Tuberculosis (TB) is an infection typically caused by mycobacterium tuberculosis. It normally affects the lungs but can be found in other areas of the body. Most cases do not have symptoms, known as latent TB – around 10% of latent infections will progress to active disease. Common symptoms of active pulmonary TB include:
- Chronic cough
- Night sweats
- Weight loss
Diagnosis of active TB is based on chest X-ray and Ziehl-Neelsen staining of sputum (as mycobacteria do not grow on other stains). Latent TB can be identified using a tuberculin skin test.
Treatment is complex and long term:
- Isoniazid and Rifampicin are taken for 6 months
- Pyrazinaminde and Ethambutol are also taken for the first 2 months