Immunity is the natural defenses that allow one to avoid or fight off disease, infection, or other types of biological invasion of the body. Immunity is generally divided into two major types: innate immunity and adaptive immunity.
Innate immunity is nonspecific, and individuals are born with it. Physical barriers like intact skin, chemical barriers like enzymes in tears and saliva, and events like fever comprise the innate immunity.
Adaptive immunity is a response to immune system challenges. It can be subdivided into two types: passive immunity and active immunity. Passive immunity is gotten through breast milk (which contains antibodies, chemical substances tailored to fight off specific invaders), or from certain immunizations, for example a shot of tetanus antitoxin, which contain the antibody obtained from some other person or form an animal.
Active immunity results when the body makes antibodies to a pathogen because it was actually exposed to that pathogen. This can occur when one gets an illness and subsequently becomes immune, or through administration of dead or weakened pathogens in a vaccination.
The innate immune system, also known as non-specific immune system, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner. This means that the cells of the innate system recognize and respond to pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host. Innate immune systems provide immediate defense against infection, and are found in all classes of plant and animal life.
The innate immune system is thought to constitute an evolutionarily older defense strategy, and is the dominant immune system found in plants, fungi, insects, and in primitive multicellular organisms.
The major functions of the vertebrate innate immune system include:
- Recruiting immune cells to sites of infection, through the production of chemical factors, including specialized chemical mediators, called cytokines.
- Activation of the complement cascade to identify bacteria, activate cells and to promote clearance of dead cells or antibody complexes.
- The identification and removal of foreign substances present in organs, tissues, the blood and lymph, by specialized white blood cells.
- Activation of the adaptive immune system through a process known as antigen presentation.
- Acting as a physical and chemical barrier to infectious agents
Passive immunity is the transfer of active humoral immunity in the form of readymade antibodies, from one individual to another. Passiveimmunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and can also be induced artificially, when high levels of human (or horse) antibodies specific for a pathogen or toxin are transferred to non-immune individuals. Passive immunization is used when there is a high risk of infection and insufficient time for the body to develop its own immune response, or to reduce the symptoms of ongoing or immunosuppressive diseases.