What is Inflammation? An Introduction

• lnflammation or inflammatory response is defined as the biological response of the immune system especially vascular tissues to harmful stimuli, such as microorganisms or other foreign substances.
• It occurs when tissues are injured by microbes, trauma, toxins, heat, or any other cause and hence is the body’s normal protective response against infection or injury.

• It involves the migration of the cells of the immune system to the site of injury or infection which result in the four characteristic signs of inflammation- warmth, redness, swelling and pain.
• Neutrophils are the primary inflammatory cell recruited in the first 24 hours.  Macrophages become predominant after 48 hours.
• Inflammation may have many different causes, the most common being:

1. Pathogenic bacteria, viruses or fungi
2. External injuries like scrapes or foreign objects
3. Effects of chemicals or radiation

• The major events that take place in sequence during an inflammatory response following microbial entry are as follows:

1. Vasodilation due to release of vasoactive substances from the damaged tissues including histamine, bradykinin, and prostaglandins. These cause narrow blood vessels in the tissue to expand, allowing more blood to reach the injured tissue. For this reason the inflamed area turns red and become hot.
2. Increased vascular permeability: It is seen in venules, in response to inflammatory mediators released. It is caused by the contraction of the endothelial cells causing larger intercellular gaps.
3. Leakage of plasma proteins through blood vessels into the tissues, causing swelling and edema. This helps to isolate the foreign substance and prevent further contact with other body tissues.
4. Leucocyte extravasation

(a) Margination: With the decrease in blood flow rate, leucocytes flow nearer to the vessel wall.

(b) Rolling on endothelium:  They roll along the surface of the endothelium and line-up along the vascular endothelium. With the help of endothelial cell adhesion molecules, they ultimately adhere to the endothelium.

(c) Extravasation: Once adhered, they move out of the blood vessels into the extravascular space.

(d) Chemotactic migration to the inflammation site. They are attracted by complement factors and leukotrienes, amongst others (e.g. TGF; VEGF etc) to the site of inflammation.

4. Engulfment of microbes and dead material by phagocytes by the process of phagocytosis.
5. Destruction of the microbes and initiation of healing

• However, inflammation is not always protective in nature. Sometimes it may produce injurious consequences to host tissues. For example:

1. Rheumatoid arthritis, where many joints throughout the entire body are permanently inflamed.
2. Psoriasis, a chronic skin disease.
3. Inflammation of the bowel like Crohn’s disease or ulcerative colitis.

These diseases are called chronic inflammatory diseases, and can last for years or even a lifetime.

• Inflammatory responses are mostly beneficial but are associated with pain, redness, heat, and swelling and can also cause tissue damage. Tissue damage is caused to some extent by complement and macrophages but mostly by neutrophils and their products.
• The aims and purposes of the inflammatory reaction are:

1. To localize the foreign agent or the injury to its site of entry or development.
2. To destroy the foreign agent or to remove its fragments and to clean the damage caused by an injury.
3. To initiate regeneration of tissue that has been destroyed by injury.
4. To repair the damage done and to fill up any gaps in the tissues by a scar.

References

1. https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072482/
2. https://dundeemedstudentnotes.wordpress.com/2015/11/15/acute-inflammation/
3. http://www.simplynotes.in/medical/inflammation-definition-causes-types/
4. Lydyard, P.M., Whelan,A.,& Fanger,M.W. (2005).Immunology (2 ed.).London: BIOS Scientific Publishers.
5. Parija S.C.(2012). Textbook of Microbiology & Immunology.(2 ed.). India: Elsevier India.
6. Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi : Jaypee Brothers Medical Publishers.