Immune system

The skin is our interface with the surrounding environment and also the first line of defence of our immune system. The immune system is the defensive and protective mechanism of the body. The body fights off enemies trying to penetrate through the protective barrier of the skin. So early defence crucially depends on proper functioning of the skin. Inflammation and immune reactions work together to protect the body from attack both from outside and inside (e.g. through tumour cells). In a complicated interaction between processes in the vessels, messenger substances and inflammation cells, inflammation reactions repair defective tissue and support the defensive responses of the immune system.

The skin is constantly exposed to biological, chemical and physical attack from outside. The main task of the skin’s immune system is to ward off these harmful agents and influences or to render them harmless. The biological agents include such things as viruses, bacteria, fungi and parasites, the physical ones solar radiation, heat and cold. Chemical substances can cause specific hypersensitivity reactions, e.g. contact allergies. However, immunological responses may not only fight off harmful influences from outside, but can also trigger hypersensitivity reactions such as allergies and auto-immune diseases, i.e. immune responses directed against the body’s own substances. Common types of allergy are hay fever, contact allergies or intolerance to substances contained in food or medication.

The immune system of the skin consists of various cells and messenger substances involved in the regulation of immune and hypersensitivity responses between those cells. The cells include the Langerhans’ cells, which present antigens, and the T and B lymphocytes, which are specific white blood corpuscles.

The Langerhans’ cells make up about 3 – 5 % of the cells of the epidermis and are located in the intermediate cell layer of the epidermis. The Langerhans’ cells are able to detect and absorb pathogens (agents that cause illnesses) or chemical substances. They migrate through the epidermis into the dermis, and from there are transported via the lymphatic system to the peripheral lymph nodes. There, the alien material is either destroyed, or fragments of it are presented by the Langerhans’ cells to the T lymphocytes. This activates the T lymphocytes, which release a number of messenger substances, the cytokines. The cytokines control the growth, function and differentiation of cells. When activated in this way, the T lymphocytes divide and are specifically prepared to respond to the antigen. On renewed contact with the antigen, they are then able to respond even faster and more effectively. The T lymphocytes also present the antigens (fragments of harmful agents) to which they have reacted to the B lymphocytes, and activate them. The B lymphocytes then produce soluble antibodies, the immunoglobulins, and release them into the blood. The immunoglobulins recognise and bind the alien antigen and initiate a cleaning-up process.

The specific lymphocytes and immunoglobulins are returned to the skin via the blood system. The earlier the immune response takes place, the less harm is caused to the body. In the case of allergies, the information about the antigen is stored for life in specific immune cells (“memory cells”). Renewed contact with the antigen, e.g. the nickel used in fashion jewellery, immediately triggers an intolerance response of the skin, or in the case of pollen an allergy of the mucous membranes and respiratory organs.

For the skin, two types of the four immunological reactions are of special importance. The early reaction type (type 1, humoral allergy) occurs within seconds or minutes, and may recur after another 4 – 6 hours. This type can be observed in nettle rash (urticaria) and allergies of the respiratory organs (hay fever, asthma). It can be triggered by such things as insect stings (bees, wasps), foodstuffs or medication. The delayed reaction type (type 4, cell-presented allergy) occurs after 12 – 72 hours, provided it has been preceded by a sensitisation phase over 5 – 21 days. It can be caused e.g. by contact allergies (nickel in fashion jewellery), tuberculin, or medication.