The immune system is the biological defense system of living beings that prevents tissue damage caused by pathogens. It removes microorganisms and foreign substances that have entered the body and is also capable of destroying the body's own cells that have become defective. The immune system is a complex network of different organs, cell types and molecules. It is of great importance for the physical integrity of living beings, because practically all organisms are constantly exposed to environmental influences.

When harmful microorganisms enter the body, diseases can develop. Typical pathogens are: Bacteria, viruses and fungi, as well as single-cell parasites (e.g. protozoa and plasmodia) and multi-cellular parasites (e.g. tapeworms). Changes inside the body can also threaten the existence of a living being: if normal body cells lose their healthy function over time, they usually die and have to be degraded (necrosis) or degrade themselves in the process (apoptosis). In rare cases, they can also degenerate pathologically and lead to the development of cancer.

All living beings - whether animal, plant or human - have protective functions. Even simple organisms possess a defense mechanism; the immune defense. Vertebrates have also developed a complex, adaptable, so-called adaptive immune defense, which protects them even more effectively against pathogens.

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