Active Immunity

Active Immunity Definition

An 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, active immunity requires a process of training immune cells to recognize and counteract foreign bodies. 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 how these bodies present themselves, and prepare antibodies, or proteins meant to encapsulate and identify these foreign organisms.

To do this, 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 into which the key fits perfectly. To quickly encapsulate and identify many foreign bodies at once, numerous antibodies are released by the immune system. They travel through the blood stream into various parts of the body, helping the immune system find and digest foreign invaders.

In an 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.

A 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 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.

Examples of Active Immunity

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 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 an 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 this 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 vaccine.

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 make up 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 for protecting the entire population.

Vaccines against many diseases are now being developed along the 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.

  • Vaccine – A serum containing antigens of a certain disease, meant to train the immune system to fight off an infection.
  • Immune System – A system of cells in an organism which work together to find and destroy foreign bodies that might cause harm.
  • Antigen – A protein attached to the surface of a foreign body which can be recognized by proteins on immune cells.
  • Antibody – A protein produced by the immune system meant to incapacitate and tag foreign bodies for disposal.


1. A baby receives antibodies from its mother that protect it from certain diseases. These antibodies are produced in the mother and passed to the baby through the umbilical cord. Which type of immunity is this?
A. Active
B. Passive
C. Immune reaction

Answer to Question #1
B is correct. This is an example of a passive immunity. When the mother is producing the antibodies, the baby never learns how to produce those antibodies. Therefore, if the baby were to become infected after the mother’s antibodies have worn off, there would no longer be an immunity. However, it is thought that the mother’s antibodies simply supplement the baby’s active immunity until it is strong enough to fight its own battles.

2. In a virus like HIV, the virus infects primarily the cells of the immune system. Why is it difficult to develop an active immunity to this type of virus?
A. It mutates faster than most viruses.
B. The immune system has no way to destroy its own cells.
C. Without the immune cells, the antigens cannot be learned, therefore no immunity can be generated.

Answer to Question #2
C is correct. While A is true of the HIV virus, the main reason it is so hard to develop an active immunity to is that in doing so, you must destroy your own immune system. Whenever a cell is infected by a virus, the entire cell will be destroyed by the actions of the virus. If these cells are immune cells, not only is your immune system weakened by losing these cells, but the immune system will begin to attack itself as the infection worsens. AIDS is the outcome of an immune system destroying itself, and leaving the body open to infection.

3. A bone marrow transplant involves transplanting many cells from the immune system of one person into another person. If the transplant recipient becomes resistant to a disease after the transplant, are they experiencing an active immunity or passive immunity?
A. Active
B. Passive
C. Pass

Answer to Question #3
A is correct. While it may seem like the immunity was “injected” into them, like a passive immunity, transporting the immune cells, and not just the antibodies, means the cells will be producing their own antibodies with the energy of the body. Therefore, this can be considered an active immunity.
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