What Is Skin Inflammation?

The inflammatory response is a useful defensive reaction of the living and blood-supplied tissues to an attack. There are three different types of aggressions:

• Physical aggression: Trauma, radiation, heat, cold;

• Chemical aggressions: Acids, Alkali, Enzymes;

Immunological and infectious aggressions: Bacteria, viruses, parasites, allergic reactions.

The skin is the heaviest and largest organ of the human body: it weighs on average 10 kg and measures 2m². This organ performs several tasks, including protecting the body from external attacks. It is the only organ that is in direct contact with the outside world.

The skin has a specialized immune defense system, which was named the "skin immune system" by Bos in 1986. The cutaneous immune system is composed of different cells that protect the organism: cells resident in the skin (keratinocytes, Langerhans cells...) and cells produced during inflammation (polynuclears, T lymphocytes...).

When the skin is attacked, a complex defense mechanism is set in motion: the inflammatory response.

In this process, the body resorts to actors capable of fighting foreign substances. In fact, "sentinel cells" ensure that the white blood cells are warned. These immune cells are constantly in the tissues, even if they are not injured or infected. White blood cells, which also circulate in blood and lymph, are called leukocytes and are capable of detecting foreign substances.

The first immune cells to come into play are the mast cells, dendritic cells and macrophages. These cells have receptors on their membrane surface that can identify the type of attacker. This identification initiates the inflammatory response.

After the foreign body comes into contact with the appropriate receptor, the warning cell releases chemical mediators that act as warning signals to the entire immune system. Some of these chemical mediators, such as histamine released by mast cells, increase the dilation and permeability of vessels near the injured site. This leads to redness, swelling and heat development – all clinical signs related to the skin's inflammatory response.

In addition, the cells may also emit other inflammatory mediators called cytokines. These are able to attract other fighting cells, such as macrophages, to the site of infection for reinforcement. These quickly neutralize the foreign microorganism by "phagocytizing" it. In the cytoplasm of the macrophages, the foreign microorganism is then broken down by enzymes and digested properly. The residues from this digestion are rejected, but no longer pose a threat to the body.

Four clinical signs were defined by the researcher Celsius. These are recognizable after inflammation of the skin or subcutaneous tissue:

• Redness,

• Pain,

• A swelling (swelling, edema),

• A feeling of warmth in the affected area.

The redness and the feeling of warmth are caused by the vasodilatation in the injured area. The swelling or edema occurs due to the plasma leakage associated with the increased permeability of the vessels. The edema then compresses the local nerve fibers and causes painful sensations.

The physician Claudius Galen later added a fifth clinical sign: loss of function. This is because inflamed skin can no longer properly perform its function as a barrier.

By the way, if the inflammation comes from the inside, these signs are present but not visible. Only pain is felt, which often becomes more intense at night.

• Barrientos S, Stojadinovic O, Golinko MS, Brem H, Tomic-Canic M. Growth factors and cytokines in wound healing. Wound Repair Regen. (2008).

• Eming SA, Krieg T, Davidson JM. Inflammation in wound repair : molecular and cellular mechanisms. J Invest Dermatol. (2007)

• Wilgus TA, Roy S, McDaniel JC. Neutrophils and Wound Repair : Positive Actions and Negative Reactions. Adv Wound Care (New Rochelle). (2013)