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 small viruses to large parasites, and is able to differentiate these substances from the healthy cells and tissues of the organism under normal conditions.
Pathogens can evolve and adapt rapidly to evade detection and attack by the immune system. To be successful in combating 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 eukaryotic organisms, such as plants and insects, inherited simple 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 recognize conserved components in many different pathogens, as well as damaged or stressed cells emitting warning signals. The innate immune system can respond quickly to a wide range of pathogen invasions but cannot generate lasting immunity to a particular pathogen. This immune system is the primary defense system in most organisms.
Adaptive immune responses have a high degree of antigen specificity, identifying "non-self" antigens through a process called antigen presentation. The specificity of antigens makes the response more targeted. This targeted response is carried out by immune memory cells within the body. If a pathogen invades the body more than once, these specific memory cells can quickly eliminate the pathogen.
