Active Immunity – Definition, Types, Examples

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Definition of Active Immunity

  • Active immunity is a resistance to disease resulting from the immune system’s production of antibodies. Active immunity, in contrast to passive immunity, in which antibodies are injected into an organism during pregnancy or artificially acquired, needs a process of training immune cells to recognise and combat foreign substances.
  • Activite immunity results from pathogen exposure. The surface indicators of a pathogen serve as antigens, which are antibody binding sites.
  • Antibodies are Y-shaped protein molecules that can exist independently or bind to the membrane of certain cells.
  • The body does not maintain an initial supply of antibodies to combat an infection. Clonal selection and expansion is the procedure that generates sufficient antibodies.

Features of Active Immunity

  • Active immunity requires pathogen exposure or pathogen antigen exposure.
  • Antibodies are produced in response to antigen exposure. These antibodies identify a cell for death by lymphocytes, which are specialised blood cells.
  • T cells (cytotoxic T cells, helper T cells, memory T cells, and suppressor T cells), B cells (memory B cells and plasma cells), and antigen-presenting cells are engaged in active immunity (B cells, dendritic cells, and macrophages).
  • There is a delay between antigen exposure and the development of immunity. The initial exposure causes what is known as a primary response. If a person is exposed to a virus a second time, the response is significantly more rapid and potent. The term for this is secondary reaction.
  • Active immunity is long-lasting. It might last for years or a lifetime.
  • Few adverse effects are associated with active immunity. It has been linked to autoimmune disorders and allergies, but normally causes no issues.

Types of Active Immunity

There are two varieties of Active Immunity:

1. Naturally active immunity

  • When the immune system of an individual comes into contact with an antigen, such as a pathogen that causes an illness, naturally active immunity occurs.
  • The adaptive immune system responds to the infection by creating antibodies and activated T cells that neutralise or eliminate it.
  • The immunity produced can be permanent, as in the case of measles or chickenpox, or temporary, as in the case of influenza.

2. Artificially Active Immunity

  • Vaccination induces artificially active immunity; that is, the host is deliberately exposed to a foreign substance to stimulate the development of antibodies and activated lymphocytes.
  • A vaccine contains antigens that can be manufactured in a variety of methods, including through the use of deceased microorganisms, living, weakened (attenuated) microorganisms, genetically altered organisms or their products, or inactivated bacterial toxins (toxoids).

Types of vaccines used for Artificially active Immunity

There are four types of traditional vaccines:

  • Inactivated vaccines are formed of microorganisms that have been rendered noninfectious by chemicals and/or heat. Vaccines against influenza, cholera, plague, and hepatitis A are examples. Most of these immunizations will likely require booster injections.
  • Live, attenuated vaccinations consist of microorganisms that have been cultured under circumstances that render them incapable of causing disease. These effects are more long-lasting, although they may require booster doses. Diseases such as yellow fever, measles, rubella, and mumps are examples.
  • Toxoids are inactivated microorganism-derived toxins utilised before to exposure to the microorganism’s toxin. Tetanus and diphtheria are two examples of toxoid-based vaccinations.
  • Subunit, recombinant, polysaccharide, and conjugate vaccines are formed of small bits or components of a harmful organism. The subunit vaccination against the Hepatitis B virus is exemplary.

In addition, the following vaccines are relatively new:

  • Outer Membrane Vesicle (OMV) vaccines include the outer membrane of a bacterium but none of its genetic material or internal components. Therefore, they should ideally induce an immune response that is efficient against the original bacterium without posing a danger of infection.
  • Genetic vaccines provide antigen-coding nucleic acid to host cells, which then synthesise the antigen and elicit an immunological response. This category of vaccines consists of DNA vaccines, RNA vaccines, and viral vector vaccines, which differ in the chemical type of nucleic acid and the method of delivery to host cells.

Mediators of active immunity

Humoral immunity and cell-mediated immunity mediate active immunity. These two forms of immunity are mediated by various immune system components and kill different types of infections in different ways.

Humoral immunity

  • It is mediated by antibody molecules in the blood and mucosal secretions.
  • B cells are the subgroup of lymphocytes that release antibodies.
  • Antibodies identify microbial antigens, combine precisely with the antigens, counteract the infectiousness of microorganisms, and target bacteria for elimination via a variety of effector mechanisms.
  • The primary defence mechanism against external microorganisms is humoral immunity.

Cell-mediated immunity

  • It is mediated by CTLs and activated TH cells. TH cells release cytokines that activate phagocytic cells, allowing them to phagocytose and destroy germs.
  • Against a wide range of bacterial and protozoan infections, this sort of cell-mediated immune response is very crucial.
  • CTLs serve a crucial function in the destruction of virus-infected and tumour cells. They eliminate changed self-cells.

Active Immunity Process

  • Certain immune system cells respond to proteins on the surface of bacterial cells, viruses, and other foreign organisms to generate active immunity.
  • The form of these proteins is “learned” through the production of a protein that can envelop the antigen on the surface of the foreign body.
  • If the foreign body antigen is a protein key, then the immune system can produce a protein lock that precisely matches the key.
  • Numerous antibodies are secreted by the immune system in order to rapidly encapsulate and recognise multiple foreign substances at once.
  • They travel throughout the body via the bloodstream to aid the immune system in locating and digesting foreign invaders.
  • With active immunity, illness resistance can be maintained for an extended period of time. Once the immune system has learnt how to generate an antibody, it may do it again and again.
  • Some of the immune system’s antibodies can be connected to immune cells that scan the body for external intruders.
  • This sort of active immunity is significantly more successful in the long run, especially if the initial infection is survivable.
  • Subsequent infections will be far less hazardous since the active immunity will destroy the pathogen before it can cause extensive harm to a large number of cells.

Examples of Active Immunity

Smallpox Immunity in Cow Maidens

  • Edward Jenner’s contribution to the development of the first effective vaccination in the 1790s was a monumental accomplishment in medical science.
  • Jenner noted that cow maidens have an unusual resistance to a deadly sickness that was becoming epidemic.
  • The cow maidens, having been exposed to cowpox, the animal equivalent of smallpox, would not exhibit the dramatic symptoms of the majority of sufferers.
  • Smallpox would typically manifest as tiny boils all over the body. Cow maidens had none of these indications.
  • Active immunity against smallpox afforded them resistance to the disease.
  • Being related to the smallpox virus, the cowpox virus has a similar structure and antigens.
  • When exposed to a cow with cowpox, the cow maidens frequently contracted the virus.
  • Cowpox has a substantially greater survival rate and fewer severe symptoms than smallpox. In this infection, the immune system would learn to manufacture antibodies against the cowpox antigen.
  • The immune system would maintain some of these antibodies in order to detect the virus in the future after the infection had subsided.
  • Due to the similarity between the antigens of smallpox and cowpox, cow maidens with active immunity to cowpox would also exhibit active immunity to smallpox.
  • Infected with the smallpox vaccination, the maidens would exhibit minimal to no symptoms while the virus was eliminated from their bodies.
  • By witnessing these peculiar occurrences, Jenner was able to recreate the effect by infecting humans with cowpox, so immunising them against the more lethal smallpox virus.

Modern Day Active Immunity

  • Today, the intricate mechanisms by which the immune system generates active immunity are considerably more understood.
  • In 1955, for example, Jonas Salk created the polio vaccine. Salk spent years analysing the structural makeup of numerous poliovirus strains in order to understand how to best immunise against them.
  • Salk eventually figured out way to eliminate the virus while preserving its vital antigens.
  • Instead of discovering a “substitute” virus to develop an analogous active immunity, Salk discovered out how to utilise a virus, even a highly contagious and lethal one, in perfectly safe ways to defend the entire population.
  • Vaccines against numerous diseases are currently being developed in the same manner as Salk’s research. Vaccines have been developed to stimulate active immunity against viruses, bacteria, and other pathogens.
  • Certain vaccinations, such as an HIV vaccine and a cancer vaccine, continue to be problematic for modern research.
  • The challenge with vaccinations for such disorders is that they frequently have characteristics that are indistinguishable from those of healthy cells.
  • This makes it difficult for researchers and the immune system to discriminate between good and harmful cells.


  • Microbiology and Immunology 2nd Edition by Shubash Chandra Parija
  • Kuby Immunology 7th Edition
  • David Baxter, Active and passive immunity, vaccine types, excipients and licensing, Occupational Medicine, Volume 57, Issue 8, December 2007, Pages 552–556,
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