Inflammation and Healing

What is inflammation?

There are three lines of defence in the body, and these are in the order of which they act:
  1. Epithelial barriers
  2. Groups of cells, tissues and organs that work together to protect the body: non-specific or innate
  3. Specific or adaptive, which can differentiate between specific microorganisms and responds accordingly, and can produce a heightened response upon re-exposure (B and T lymphocytes)
Inflammation results in an exudate, which can either be:
  • Serous, meaning there is a lot of fluid and decrease in protein
  • Purulent, meaning it contains pus (neutrophils)
  • Fibrinous, meaning there is a lot of fibrin
  • Haemorrhagic, meaning it is due to severe tissue injury and damage to vessels which causes red blood cells to leak from the capillaries
In acute inflammation, involved cells include neutrophils, which phagocytose, basophils and mast cells, which release histamine, and eosinophils, which are involved in allergic reactions and parasitic invasion. Each of these cells is known as a granulocyte. There is both a vascular and cellular phase:

Vascular phase: ↑ vasodilation causing ↑ blood flow to injured tissue, ↑ capillary permeability (allows fluid and blood proteins to move into the interstitial fluid), ↓ blood flow speed

Cellular phase: margination of blood cells (leukocytes, e.g. neutrophils) occurs where they accumulate near the blood vessel wall. The line of leukocytes along the vascular wall causes an adhesion reaction to occur between the vessel wall and the leukocytes, and eventually the leukocytes migrate into the extravascular space. An inflammatory exudate, one of the above four types, is then formed from oedema.

Acute inflammation presents with redness (vasodilation), swelling (increased capillary permeability), heat (vasodilation), pain (swelling, bradykinin, prostaglandins and cellular mediators) and loss of function (swelling and pain). Systemically, fever can occur.

Acute inflammation can have one of four outcomes, which are summarised below:

  1. Resolution, where tissue damage is minimal
  2. Abscess, where there is a local accumulation of oedema, necrotic debris and pus
  3. Scarring
  4. Chronic inflammation, due to tissue destruction
In chronic inflammation, there is a response to a longer term injury (persistent infection) which lasts from weeks to years. It involves different cells to acute inflammation known as agranulocytes. The cells involved are monocytes and lymphocytes. It has only two outcomes:
  1. Scarring
  2. Persistent chronic inflammation
The outcome of a tissue from inflammation also depends on the tissue that is affected. For example, epidermis, gastrointestinal epithelium, bronchial epithelium, bone marrow, liver and kidney can regenerate. The first four types of cells are known as labile cells, have a short lifespan and are continuously reproducing from stem cells. The last two are stable cells, only regenerating when triggered by stress or injury. However, the brain, cardiac muscle and skeletal muscle cannot regenerate, only by fibrosis. This makes them permanent cells, as they are unable to regenerate and are differentiated from stem cells with no remaining pool of stem cells. Injury to such cells always results in scarring.

How can we heal?

Healing of narrow wounds is known as 'healing by first intention'. 'Healing by second intention' occurs more in broad wounds with widely separated edges. Healing by first intention occurs for wounds such as surgical incisions, where the wound edges are closely positioned:
  1. Initial haemorrhage → clot formation
  2. Neutrophil infiltration → exudate formation
  3. Inflammation due to 1. and 2. (day one)
  4. Angiogenesis (formation of new blood vessels)
  5. Mitosis of endothelial cells
  6. Fibroblasts repair the injury (steps 4. to 6. occur during days three to seven)
  7. Scar (day thirty)
When healing by second intention, there is normally necrosis of the epidermis and dermis, e.g. due to burning of ulcers, and the scar is much wider than the narrow scar from first intention healing.

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