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Inflammations are natural processes by which harmful invaders, usually microorganisms such as bacteria, fungi or viruses are repelled to prevent an infection. The inflammatory response plays three important roles in fighting infections: it delivers effector molecules and cells, forms a physical barrier and stimulates tissue repair.

The inflammatory process is a complex process in which many cells of the immune system are involved. A malfunction of inflammatory processes can lead to chronic inflammation or allergies, which can damage the body extensively. Triggers for an inflammatory response can also, besides infection, be an injury to tissue and the uncontrolled death of cells.

Cytokines as mediators of the inflammatory response

In an inflammatory response triggered by microorganisms, receptors on the cells of the immune system (e.g. macrophages) recognize the characteristic, highly conserved surface structures of, for example, bacteria. If these foreign structures are detected, it leads to the release of messengers, including the so-called cytokines that can stimulate other cells to divide, and can activate or attract them (chemotaxis). Upon the release of cytokines and mediators of inflammation, the body reacts with inflammation, which is defined by four characteristics: heat, pain, redness and swelling.

One of the reactions to the release of cytokines is, for example, the dilation of blood vessels and an increase in the permeability of the blood vessels, which provides easy access to the tissues for the cells the immune system has attracted. The entry of the cells and effector molecules results in inflammatory pain.

Phagocytic cells (e.g. macrophages, dendritic cells and neutrophils) are some of the major cell types in the first phase of an inflammation, which can surround and kill the pathogen. However, other cells of the innate immune system (cells of the innate immune system identify bacteria nonspecifically to the stuctures which occur in almost all bacteria) play a role.

During the later phase of the inflammatory response, the adaptive immune cells (the cells of the adaptive immune system can identify bacteria on the basis of variable receptors specific to certain structures [epitopes, antigens] of bacteria) also play a role.  

The adaptive immune system allows for a targeted response to specific pathogens

Upon activation of the cells of the adaptive immune system (e.g. lymphocytes), the innate immune system again plays a decisive role, since it allows the lymphcytes access to tissue and the so-called antigen-presenting cells (dendritic cells and macrophages) present components of pathogens (antigens) to the cells of the immune system. When T-lymphocytes (T cells) identify these antigens through their variable receptors, they are activated and multiply themselves through cell division.

A special feature of the immune system is that it can selectively identify specific pathogens and can react directly to them. Another important characteristic of the adaptive immune system is that it has a so-called immunological memory. Certain cells can, after contact with a pathogen, exist for many years in the body and can trigger a rapid, targeted and effective immune reaction by a further contact with the same pathogen (reinfection). Nowadays, medical science makes use of this characteristic with vaccines. However, if it malfunctions, this characteristic can also lead to allergies and autoimmune diseases.

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