What is Active immunity? | Definition, Overview, and Example.

 Active Immunity Definition

Active immunity is a resistance to disease through the creation of antibodies by the immune system. As opposed to passive immunity, where antibodies are injected into an organism during pregnancy or they are artificially acquired, active immunity requires a process of training immune cells to recognize and counteract foreign bodies.

Active Immunity Overview

Typically, a bacteria or virus enters an organism and starts causing damage through its reproductive activities. The damage being done to cells releases a signal to immune cells that something is wrong. The immune cells surround the foreign bodies and digest them, to remove them from the organism. At the same time, the immune system learns which proteins are present on these invaders, and prepares antibodies, or modified proteins, which encapsulate and identify these foreign organisms.

Immune cells are “trained” to recognize these invaders with the lymph nodes and other immune system tissues. Once the original infection is cleared out, the immune cells retain their training in the form of antibodies bound to their cell membranes. Thus, when they encounter the invader the next time, the antibody will automatically bind to the proteins on the surface of the invader. This gives the organism a naturally acquired immunity, rather than a passive immunity conferred by the introduction of antibodies.

Several autoimmune diseases are caused by a malfunction of the active immunity systems. This is rather common because the process is imperfect. Sometimes, the immune cells will learn to identify proteins the body produces as “invaders”. Then, when immune cells encounter cells of the body with specific proteins, they will attack. This is the basis of all autoimmune diseases.

Active Immunity Examples

Smallpox Immunity in Cow Maidens

The development of the first successful vaccine, back in the 1790s, was an enormous advance to medical science made possible by Edward Jenner. Jenner observed that cow maidens had a peculiar resistance to a terrible disease that was becoming an epidemic. The cow maidens, having been exposed to the animal form of smallpox (known as cowpox), would not show the dramatic symptoms of most patients. Typically, smallpox would present itself with small boils all over the body. The cow maidens did not show these symptoms. Their resistance to the disease was provided by the active immunity they received to smallpox.

The cowpox virus, being related to the smallpox virus, has a similar shape, and also similar antigens. The cow maidens, being exposed to a cow with cowpox, would often catch the virus themselves. Unlike smallpox, cowpox has a much higher survival rate and less brutal symptoms. The immune system would learn to produce antibodies to the cowpox antigen in this infection. Once the infection had passed, the immune system would retain some of these antibodies to help detect the virus in the future. Because the antigens of smallpox and cowpox are so similar, cow maidens with active immunity to cowpox would also show an active immunity to smallpox. Thus, once infected with the smallpox vaccine, the maidens would show few to no symptoms as the virus was cleared from their systems.

By observing these curious phenomena, Jenner was able to replicate the action by infecting people with cowpox, thus giving them an active immunity to the more deadly smallpox virus.

Modern Day Active Immunity

Today, the complex processes by which the immune system is able to create an active immunity are much better understood. For instance, Jonas Salk developed the polio vaccine in 1955. For years, Salk studied the structural makeup of various strains of polio, in order to determine how best to vaccinate for them. Salk eventually learned how to successfully kill the virus, while leaving the important antigens intact. Instead of finding a “substitute” virus to produce an equivalent active immunity, Salk had figured out how to use a virus, even a very contagious and devastating one, in ways that were completely safe to protect the entire population.

Vaccines against many diseases are now being developed along the same lines of Salk’s work. Vaccines have been made to induce active immunities for viruses, bacteria, and other foreign bodies. Modern research still struggles with certain vaccines, such as an HIV vaccine and a cancer vaccine. The problem with vaccines for diseases like these is that they often present themselves in ways that are indistinguishable from healthy cells. This makes it hard for both the researchers and the immune system to distinguish which cells are bad and which are good.

Active Vs Passive Immunity

The difference between active and passive immunity is simply where the antibodies came from. In active immunity, the immune cells of the body recognize foreign particles and cells and create antibodies to combat them. Passive immunity, on the other hand, simply gives an organism the correct antibodies to combat germs and pathogens. Passive immunity is most commonly seen in pregnancy when a mother’s antibodies pass to the baby and protect it. The baby’s active immunity is not developed yet, so it needs its mother’s antibodies.

Active Immunity Process

To create active immunity, certain cells in the immune system respond to proteins on the surface of bacterial cells, viruses, and other foreign bodies. The shape of these proteins is “learned” by creating a protein which can surround the antigen on the surface of the foreign body. By analogy, if the foreign body antigen is a protein key, the immune system can create a protein lock which fits the key perfectly. To quickly encapsulate and identify many foreign bodies at once, numerous antibodies are released by the immune system. They travel through the bloodstream into various parts of the body, helping the immune system find and digest foreign invaders.

With active immunity, resistance to a disease can be carried on for a long time. Once the immune system has learned to produce an antibody, it can do so repeatedly. Some of the antibodies produced by the immune system can be attached to immune cells that search through the body for foreign invaders. This type of active immunity is much more effective in the long run in resisting disease, especially if the first infection is survivable. Subsequent infections will be much less dangerous because the active immunity will mean the disease gets eradicated before it can cause severe damage to a large number of cells in an organism.


Vaccine-Induced immunity is a type of active immunity in which the initial infection is produced by the injection of a dead virus or dead bacteria into a person. Although commercially produced vaccines are produced by much more complicated and stringent standards, the process is the same as the following:

In some way, the foreign bodies are “killed” in that they can no longer carry out the task of reproduction. But, they must be left somewhat intact, so that the antigens, or proteins they present on their surfaces, can still be recognized by the immune system. A serum containing these dead foreign bodies is injected into a live organism. The immune system of that organism reacts to the foreign bodies and creates an active immunity against the presented antigens.

When you are subsequently infected by the actual organism, your body quickly recognizes the antigens present and destroys the organism before it has the chance to reproduce and wreak havoc on your body. There are some unfortunate cases in which an active immunity can start to target cells of your own body. The continual immune response to your own cells is known as an autoimmune disease. Typically, the immune system only functions to protect you, but it is import to know that in certain cases, it can be a detriment.

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