Online slides of cell injury, inflammation, and repair
The inflammatory response is very familiar. TRY IT YOURSELF -- A mild example can be produced at your convenience. Firmly scrape the skin on your forearm, and then watch as redness (with possibly some mild swelling and heat) develops over the next few minutes.
Inflammation is characterized by several familiar signs, redness, swelling, heat and pain. To understand inflammation is to understand how and why these signs develop.
These signs represent a response that is programmed into your tissue. This response is one of your body's principal defense reactions, designed to anticipate, intercept and destroy invading microorganisms. Inflammation is best appreciated by understanding your body's functioning at the level of cells and tissues. Subsequent processes of tissue repair (healing) involve cell growth and division, cell movement and differentiation, and manufacture of extracellular material.
We go through life separated from our environment by a cellular boundary. This boundary --including the epidermis of our skin and the epithelia of our various mucous membranes--covers all our body's surfaces, even the internal surfaces of lungs, gut and glandular ducts. Nothing enters or leaves the body except by passing through or between the cells which form this epithelial boundary.
Whenever this epithelial boundary is broken, as it often is by scrapes and cuts and insect bites, two unfortunate things can happen. Good stuff like blood can leak out. And bad stuff like germs (microorganisms) can creep in. Plugging the leak can be fairly straightforward, with a quick patch (a blood clot) preventing significant loss of bodily substance. But a quick patch is not enough to prevent serious infection. Because germs are alive, they can grow and proliferate. So even a few microbes invading your body at the moment of injury could, if left unmolested, rapidly convert your warm, moist, well-oxygenated tissues into a thriving bacterial culture. Inflammation helps avoid this result.
Unfortunately, inflammation is uncomfortable. And inflammation can sometimes be triggered inappropriately (e.g., by allergies or autoimmune diseases). Many common medical treatments (e.g., aspirin and other "anti-inflammatory agents") are intended to relieve the discomfort and/or reduce the attendant tissue disturbance that inflammation may cause.
Any tissue or organ can become inflamed. Inflammation of a particular body part is named according to the site, with the "-itis" suffix appended (e.g., tonsillitis, appendicitis, dermatitis, arthritis, sinusitis, etc.). Thus many disease names are really just words that identify sites of inflammation. (Examples)
Tissue repair. Following inflammation, injured tissue is usually replaced by new cells and extracellular materials, with undamaged surrounding cells proliferating and migrating to fill the void. Although some tissues, especially surface epithelium, can grow back quite efficiently, complex organization is seldom matched in the regenerated site. Gaps are quickly filled by collagen fibers produced by proliferating fibroblasts. A mass of collagen which replaces tissue that has been destroyed is known as a scar. Scars in the skin appear white because collagen is colorless, and healing often creates a more compact arrangement of collagen with fewer capillaries than the surrounding tissue. Scars in other organs also exist as firm masses of collagen in which normal organ function cannot occur. For example, cirrhosis of the liver represents extensive replacement by collagen of damaged liver cells.
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CARDINAL SIGNS OF INFLAMMATION
The four cardinal signs of inflammation --the four "ORs" -- are:
(A fifth sign, loss of function, is sometimes included in this list.)
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Where do these signs come from? What is happening to your body during inflammation? As you understand inflammation, you should be able to answer the following specific questions.
- Why does tissue become red and warm around a site of injury?
- Why does inflamed tissue swell?
- What is pus, an