The immune system is a disease defense system composed of a series of biological structures and processes within an organism. It can detect a wide range of pathogens and harmful substances, from viruses to parasites, and distinguish them from the body's healthy cells and tissues under normal circumstances.
Pathogens can rapidly evolve and adapt to evade detection and attack by the immune system. To overcome pathogens, organisms have evolved various mechanisms to recognize and eliminate them. Even simple single-celled organisms like bacteria have developed enzyme systems to combat bacteriophage infections. Some eukaryotes, such as plants and insects, inherited basic immune systems from their ancient ancestors. These immune mechanisms include antimicrobial peptides (defensins), phagocytosis, and the complement system. Jawed vertebrates, including humans, have developed more complex and diverse defense mechanisms.
Innate responses are typically triggered when invaders are recognized by pattern recognition receptors. Pattern recognition receptors are key molecules of the innate immune system that can identify conserved components in various pathogens and damaged or stressed cells emitting warning signals. The innate immune system can respond rapidly to a wide range of pathogen invasions but cannot produce long-lasting immunity to a specific pathogen. This immune system is the primary defense system in most organisms.
Adaptive immune responses are highly antigen-specific and can recognize "non-self" antigens through a process called antigen presentation. Antigen specificity allows for a more targeted response, which is carried out by immune memory cells in the body. If a pathogen invades the body more than once, these specific memory cells can quickly eliminate the pathogen.
