Immunity is the way your body recognizes and protects itself from bacteria, viruses, fungi and other substances that appear foreign and harmful. These substances are called antigens and your immune system targets and destroys them. It also makes sure that it doesn’t target your own cells and tissues, which are known as self-antigens. Failure to do this is referred to as autoimmune disease.
There are two types of immunity: natural and adaptive. Natural immunity develops from infection with a disease organism or through vaccine-induced immunity. When a person with natural immunity comes into contact with the disease organism again, their immune system quickly produces antibodies to fight it off. This type of immunity is long-lasting, sometimes life-long.
The other type of immunity is adaptive immunity, which involves specialized cells that recognize, target and remember specific invaders. These cells include T cells and B cells. Memory cells are key to adaptive immunity because they allow the immune system to act faster against an invading pathogen, akin to rooting out an invasive weed in your garden.
When a pathogen invades your body, the innate immune system recognizes it by the chemical structures it contains that are unique to the invading microbe or toxin. These structural features are recognized by special receptors on the surface of your cells and by proteins in the blood that are activated when a microbe or toxin enters the body. The molecular recognition mechanisms that permit innate immunity are hard-wired responses encoded by genes and that somatically rearrange to form receptors with exquisite specificity for individual microbes or toxic molecules.