In immunology, active immunity is any immunity that the body creates for itself in response to the presence of specific harmful substances, which usually are bacteria or viruses. This can take a number of different forms. In some instances, active immunity is a natural process. At other times, active immunity can be artificially triggered.

Natural active immunity occurs when a person is exposed to harmful microbes in the environment, which usually is accidentally. Once these microbes penetrate the body’s skin, mucous membranes, or other primary defenses, it interacts with the immune system. B-cells in the body produce antibodies that help to fight against the invading microbes. Artificial immunity is the type of immunity that occurs when a person is exposed to a vaccine that contains dead, weakened, or incomplete and harmless versions of a threatening microbe. The immune system is then alerted to this particular microbe and will be able to better defend against it in the future.

Regardless of whether active immunity is gained in a natural or artificial way, the process is the same. All invading microbes have certain proteins on their surfaces called antigens. When a specialized cell of the immune system called a phagocyte eats the microbe, it delivers the antigen to a white blood cell called a helper T cell. Helper T cells then activate other cells of the immune system, which help to destroy the current infection. This process also creates memory cells — cells that become activated to fight again in the event the same microbe strikes again.

This process is described as active because the body must actively react to the presence of microbes in its internal environment. The opposite is called passive immunity. In passive immunity, the materials necessary to fight off infection are produced outside of the body and then delivered to another person. This can happen in natural passive immunity, such as a child receiving antibodies from a mother’s milk. It can also be artificial passive immunity, such as when a person receives an injection of pre-made antibodies.

Active immunity can be either cell-mediated or humoral immunity. In cell-mediated immunity, cells such as cytotoxic T cells hunt down and kill specific invaders by using the antigens on their cell membranes as identifying markers. With humoral immunity, B cells produce antibodies that bind to specific invaders, again using the antigens on their cell membranes as identifying markers. These antibodies then make it easier for white blood cells to either destroy the microbes or neutralize their negative effects. In both natural active and artificial active immunity, the body produces all the cells and materials to perform both cell-mediated and humoral immune functions.

It is important to understand that innate is different from active. Active immunity is a subset of adaptive immunity, in which the body attacks specific invaders. Innate immunity defends the body against invaders without attacking specific types. For example, lymphocytes such as neutrophils, macrophages, and natural killer cells attack foreign microbes regardless of what antigen they have on their cell membrane.