The immune system
Specialized cells, cytokines and other messenger substances, antibodies – a complex system protects tissue and organs from harmful external influences. Without immune defense foreign substances would intoxicate the body and infectious agents such as viruses and bacteria could easily overcome the human body. The immune system has to recognize harmful and dangerous substances and distinguish them from harmless substances as well as from its own tissue – this prevents cell damage.
Every organism is constantly exposed to environmental influences. Some of these are dangerous: bacteria, viruses and other microorganisms can cause disease. In addition, external substances can have toxic effects on the body. Therefore, every living creature has protective mechanisms. The skin is an external barrier, but internally cells and molecules of the immune system exert their protective function in many different ways.
Innate and acquired immunity
Animal and human beings have inherent protective mechanisms. Various immune cells can distinguish between “foreign” and “self”, and can thus rapidly recognize invading foreign organisms. They can even distinguish between “dangerous” and “harmless”. The immune cells can attack and destroy infectious intruders in due course. It is estimated that about 90 % of all infections are rapidly cleared by these innate mechanisms.
The immune system also has a learning function. Memory cells can remember what infectious agents they have seen in the past looked like. When these or similar infectious agents try to attack the body, the immune cells rapidly multiply and secrete large amounts of protective antibodies. These acquired or learned protective mechanisms and the inborn, innate protective mechanisms interact closely and thus constitute an effective protective system.
Cells and molecules
The immune system includes a number of various cell types and molecules that can influence each other. Their coordinated reaction is the basis for effective immune defense.
- Macrophages are “eating cells” that can swallow infectious agents and destroy them subsequently. During this process, they secrete messenger molecules that can activate other defence cells.
- Killer cells recognize viruses inside infected cells and can specifically destroy these virus-infected cells. In addition they can destroy tumor cells.
- Helper cells support and coordinate the immune reaction by secreting messenger substances and thus activating and strengthening other cells of the immune cells.
- Regulatory cells control the immune reactivity and prevent an overreaction of the immune system.
- Memory cells store the information of previous infectious agents and can react very quickly upon re-exposure to the same agent. Memory cells are the primary carriers of immune protection following vaccinations.
- Plasma cells produce antibodies that can protect from viruses, bacteria and bacterial toxins as well as other toxic substances.
- Antibodies are special proteins that have the ability to vary enormously. They highly specifically bind to certain structures, such as surface molecules on viruses and bacteria.
- Cytokines and interleukines are messenger substances that are built by immune cells. They stimulate and regulate other immune cells and help them to grow and act in the immune defence.
Dangerous or harmless?
One of the most important abilities of the immune system is the distinction between dangerous and harmless substances. An immune reaction should specifically and strongly fight dangerous intruders without doing harm to its own body. This is a very difficult task, because infectious agents may often use molecules and structures that look very similar to the body’s own molecules. In addition, viruses go into the cells they infect and thus try to hide.
Nonetheless, the immune system is able to focus on undesired intruders. Immune cells recognize specific markers on infectious agents but they also recognize specific cell molecules signaling to the body that these are harmless and structures wanted by the body. This allows for example specific recognition of cells infected by viruses and influenced by the viruses in a recognizable way.
The body’s defence system is finely tuned and highly effective. Nonetheless, it can be misguided, because the balance between recognizing self and altered self is so delicate. This misguidance may cause accidental attack of the body’s own cells or structures.