Skin defense systems

While happily burying ourselves in the warm sand on the beach, picking flowers in the forest, wandering barefoot on the ground and lying on the grass, we hardly think about the enormous and intense work that the skin's immune system is doing at this time. After all, myriads of microorganisms, allergens, dirt particles, sharp grains of sand and a whole range of various chemical compounds end up on the skin. Even a cosmetic cream standing on the shelf in the bathroom can be an explosive mixture of microbes and aggressive chemicals, increasing the already large load on the skin's protective systems. Yes, we have to admit that our skin usually shows amazing resilience. And yet, pimples and acne can appear on the skin, despite daily washing and wiping it with alcohol lotion, it can become red and inflamed from a "hypoallergenic" cream, itch and peel for no apparent reason. These unpleasant phenomena are based on the same protective reactions that make the skin such a reliable barrier to microorganisms. Of course, we are all interested in the skin's protective systems working well, that is, preventing external invasions, if possible without an inflammatory reaction and other visible manifestations of the fight against infection. In other words, a good immune system is one that you don't have to think about.

The skin is called the largest immune organ. And this is not surprising, because it is easier to prevent an invasion than to fight on your own territory. The skin has two defense systems - specific and non-specific. From the point of view of evolution, the non-specific system is more ancient. It immediately reacts to any invasion and immediately begins to fight. The main cells of the non-specific immune system of the skin in the epidermis are keratinocytes and Langerhans cells, in the dermal layer - macrophages. Macrophages not only destroy the enemy, but also coordinate the activity of all cells of the immune system. A macrophage can recognize bacterial cells, since their membrane is built of special polysaccharides that are not found in the body of animals. As soon as a macrophage comes into contact with a polysaccharide of a bacterial wall (or a substance similar to a polysaccharide of a bacterial wall), it is immediately activated and begins combat actions against the intruder.

The specific immune system is different in that it must first recognize the intruder and then remember it in order to find and destroy it later. This ability to remember intruders sometimes leads to trouble. The fact is that the cells of the immune system remember not the entire molecule, but only a small part of it (which acts as an identity card). Macrophages react to any intruder, and leukocytes (cells of specific immunity) remember each molecule individually. In this sense, macrophages are like law enforcement officers who only need a passport. And leukocytes are like criminal investigators who must check identity documents and make sure that the person in front of them is not a criminal. The problem is that in the world of molecules, many substances, different in their chemical nature, have the same "identity cards". And when the immune system again and again deploys the entire arsenal of armed struggle against imaginary intruders, an allergic reaction develops.

The most serious danger of penetration of intruders into the skin occurs when the skin is damaged. Therefore, the main alarm signal for the skin's protective systems is the fragments of cell membranes that inevitably appear when skin cells are destroyed, regardless of the cause. Substances structurally close to steroid hormones, prostaglandins, are synthesized from the fragments of cell membranes. Prostaglandins control the local inflammatory reaction. Macrophages are drawn to the site of damage, absorbing bacteria and other foreign substances, and also release signal molecules that call other cells for help. Prostaglandins and substances released by macrophages cause dilation of the blood vessels of the dermis - the skin turns red. Tissue fluid and white blood cells (leukocytes) begin to emerge from the dilated blood vessels - swelling and thickening of the skin occurs. Biologically active substances produced by all these cells act on the nerve endings of the skin, causing pain and itching.

Every war is destructive, and the war waged by the immune system is no exception. The cells involved in the fight against the intruder produce many toxic molecules that damage the skin. Some scientists compare leukocytes to nuclear reactors, both useful and dangerous. Leukocytes produce a huge number of aggressive and toxic molecules that are necessary for the destruction of the microbial cell. These molecules are formed as a result of a chain reaction that is difficult to control and can get out of control at any moment. Toxic substances produced by leukocytes can destroy them and damage surrounding tissues.

If the immune system's resistance to the intruder is proportionate to the scale of the invasion, everything soon returns to normal. Blood vessels narrow and their walls become less permeable, cells spread and are carried away by the bloodstream, biologically active substances are inactivated, and restoration work begins at the site of destruction.

Disharmony and disorder in the activity of immune system cells leads to excessive damage to the skin, allergic reactions, chronic dermatitis. If the protective reaction was insufficient, the inflammatory reaction is delayed, and microorganisms can encapsulate in the deep layers of the skin, periodically renewing attacks.

Normalization of the immune system with the help of cosmetic products with immunomodulatory action is one of the most interesting and promising areas in cosmetology.

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