What is Active Immunity?

• Active immunity refers to the immune response generated by the body’s own immune system to combat foreign antigens, such as pathogens or their components. It is characterized by the production of antibodies and the activation of immune cells, specifically B-cells and T-cells.
• Active immunity can be acquired through various means. One way is through exposure to a pathogen, either through clinical or subclinical infection. When the body comes into contact with a foreign antigen during an infection, the immune system is triggered and mounts a response to eliminate the invader. This response involves the activation of B-cells and T-cells, which produce specific antibodies to neutralize the antigen and eliminate the infection. The immune system retains the memory of this encounter, allowing for a faster and more effective response in case of future exposure to the same pathogen. This is known as acquired active immunity.
• Another way to acquire active immunity is through immunization. Vaccines contain either live or killed infectious agents or their antigens. When a vaccine is administered, it stimulates the immune system to mount a response similar to what would occur during a natural infection. The immune system recognizes the vaccine antigens as foreign and produces specific antibodies against them. This immune response leads to the development of active immunity, providing protection against future infections by the targeted pathogen.
• Additionally, exposure to microbial products, such as toxins and toxoids, can also induce active immunity. These products can elicit an immune response, leading to the production of antibodies that provide protection against the specific toxins or toxoids encountered.
• The development of active immunity involves a series of immune responses. Initially, the immune system recognizes the foreign antigens and activates B-cells and T-cells. B-cells produce antibodies that can bind to and neutralize the antigens, while T-cells play a role in cell-mediated immunity, targeting infected cells directly. This primary immune response may take some time to develop, but once active immunity is established, the immune system retains the ability to respond rapidly and effectively to subsequent encounters with the same antigen. This results in a long-lasting immunity, which is a major advantage of active immunity.
• In contrast to active immunity, passive immunity involves the transfer of preformed antibodies from one individual to another. This can occur naturally, such as through the transfer of antibodies from a mother to her baby during pregnancy, or it can be artificially acquired through the administration of exogenous antibodies, as in the case of certain medical treatments. Passive immunity provides immediate protection but is temporary since the transferred antibodies are gradually eliminated from the recipient’s body.
• However, active immunity is not without flaws. Sometimes, the immune system may mistakenly recognize the body’s own proteins as foreign antigens, leading to autoimmune diseases. In autoimmune diseases, immune cells attack the body’s own cells that express specific proteins, causing damage and inflammation. This occurs due to a malfunction in the active immunity system, where the immune cells fail to differentiate between self and non-self antigens.
• In conclusion, active immunity is a type of immunity that arises from the body’s immune response to foreign antigens. It can be acquired through exposure to pathogens, immunization, or exposure to microbial products. Active immunity involves the production of antibodies and the activation of immune cells, leading to long-lasting protection against specific antigens. It contrasts with passive immunity, which involves the transfer of preformed antibodies. While active immunity provides valuable defense against infections, it can also lead to autoimmune diseases if the immune system malfunctions.

Definition of Active Immunity 

Active immunity refers to the immunity that is acquired when the body’s own immune system produces antibodies in response to exposure to a foreign antigen, such as a pathogen or vaccine. It provides long-lasting protection against specific antigens.

Characteristics of Active Immunity

Active immunity possesses several key characteristics:

1. Pathogen or antigen exposure: Active immunity requires exposure to a pathogen or its specific antigen. This exposure triggers the immune response and subsequent production of antibodies.
2. Antibody production: Exposure to the antigen stimulates the production of antibodies by B-cells. These antibodies serve as markers for destruction by lymphocytes, such as cytotoxic T cells.
3. Cell types involved: Active immunity involves various types of immune cells, including 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 (B cells, dendritic cells, and macrophages). These cells work together to recognize and eliminate foreign antigens.
4. Delayed response and memory: There is a time delay between exposure to the antigen and the acquisition of immunity. The initial exposure triggers a primary immune response, which takes time to develop. However, upon subsequent exposure to the same antigen, the immune system mounts a faster and stronger secondary response. This is due to the presence of memory B cells and memory T cells, which “remember” the antigen from the previous encounter.
5. Long-term immunity: Active immunity provides long-term protection. Once the immune system has generated a response and developed specific antibodies against an antigen, the immunity can last a lifetime. This long-lasting immunity is a crucial characteristic of active immunity.
6. Potential side effects: While active immunity is generally beneficial, there can be potential side effects. Sometimes, the immune system may mistakenly identify the body’s own proteins as foreign, leading to autoimmune diseases. Additionally, allergic responses can occur when the immune system overreacts to harmless substances. These side effects highlight the imperfect nature of the immune response and the possibility of immune system malfunctions.

In summary, active immunity is characterized by the requirement of exposure to a pathogen or antigen, the production of antibodies, involvement of various immune cell types, delayed response with subsequent memory response, long-term immunity, and the potential for side effects like autoimmune diseases and allergic responses.

Types of Active Immunity

There are two varieties of Active Immunity:

1. Naturally active immunity

• Natural active immunity refers to the type of immunity that is acquired when an individual is exposed to a pathogen, either clinically (resulting in symptoms of the disease) or subclinically (without noticeable symptoms). This exposure leads to the development of the disease and subsequently triggers a primary immune response, resulting in long-term immunity.
• When a microbe enters the body through the skin, mucous membranes, or other primary defense mechanisms, it interacts with the immune system. This interaction activates B-cells, which are responsible for producing antibodies that help fight the invading pathogen. The antibodies produced during the immune response play a crucial role in neutralizing the pathogen and preventing further infection.
• The adaptive immune response generated against the pathogen takes time to develop, typically taking days or weeks. However, once the immune response is established, it can provide long-lasting or even lifelong protection against the specific pathogen. This is because the immune system retains memory of the encountered pathogen, allowing for a faster and more effective response upon subsequent exposure.
• In cases where wild infection occurs, such as with the hepatitis A virus (HAV), and the individual recovers from the infection, a natural active immune response is typically generated. This immune response leads to lifelong protection against the specific pathogen, as the immune system retains the memory of the encountered antigen.
• Natural active immunity, acquired through exposure to a live pathogen and subsequent disease, provides robust and long-lasting protection. It is a fundamental aspect of the immune system’s ability to recognize and eliminate specific pathogens, forming a critical defense mechanism against future infections.

2. Artificially Active Immunity

• Artificially active immunity refers to the type of immunity that is acquired through immunization via vaccination. In this case, the immune response is intentionally stimulated by administering a vaccine containing an antigen derived from a disease-causing pathogen. This results in the production of an acquired artificial active immune response, which provides long-lasting or even life-long protection against the specific pathogen.
• Vaccination is a highly effective method that scientists have developed to manipulate the immune system. It takes advantage of the immune system’s natural specificity and its ability to generate immune responses. The basic principle of vaccination is to introduce an antigen into the body, derived from a disease-causing organism, which triggers the immune system to develop protective immunity against that organism. Importantly, the antigen in the vaccine does not cause the pathogenic effects of the actual organism.
• By introducing the vaccine antigen, the immune system recognizes it as foreign and mounts an immune response. This response involves the production of specific antibodies and the activation of immune cells, such as B-cells and T-cells. The immune system retains a memory of the encountered antigen, enabling a rapid and robust immune response upon subsequent exposure to the pathogen.
• Various types of vaccines are available to induce artificially active immunity against a wide range of microbial pathogens. These vaccines can be live vaccines, which contain weakened forms of the pathogen, or killed vaccines, which contain inactivated pathogens. Additionally, vaccines may also contain specific bacterial products. Each type of vaccine is designed to elicit an immune response specific to the targeted pathogen, resulting in long-lasting protection.
• Artificially active immunity through vaccination has been highly successful in preventing and controlling infectious diseases. It provides a proactive approach to disease prevention, equipping the immune system with the ability to recognize and neutralize pathogens before they cause illness. Vaccination programs have played a crucial role in reducing the burden of many infectious diseases and have saved countless lives worldwide.

Types of vaccines used for Artificially active Immunity

There are several types of vaccines used for artificially active immunity. Here is an overview of the different vaccine types:

1. Inactivated vaccines: These vaccines are composed of microorganisms that have been rendered noninfectious through chemical treatment or heat. Examples of inactivated vaccines include those for influenza, cholera, plague, and hepatitis A. Booster injections are often required for sustained immunity.
2. Live, attenuated vaccines: These vaccines contain microorganisms that have been cultured under conditions that render them unable to cause disease. They provide long-lasting immunity and may require booster doses. Vaccines for diseases like yellow fever, measles, rubella, and mumps fall under this category.
3. Toxoid vaccines: Toxoid-based vaccines utilize inactivated toxins derived from microorganisms. They are administered before exposure to the microorganism’s toxin. Examples of toxoid vaccines include those for tetanus and diphtheria.
4. Subunit, recombinant, polysaccharide, and conjugate vaccines: These vaccines are composed of small components or fragments of a harmful organism. They can include subunits of proteins, recombinant proteins produced through genetic engineering, polysaccharides, or conjugated antigens. An example is the subunit vaccine against the Hepatitis B virus.

In addition to these traditional vaccines, there are newer vaccine types:

5. Outer Membrane Vesicle (OMV) vaccines: OMV vaccines consist of the outer membrane of a bacterium without its genetic material or internal components. These vaccines aim to induce an immune response effective against the original bacterium without causing infection.
6. Genetic vaccines: Genetic vaccines deliver antigen-coding nucleic acid to host cells, which then synthesize the antigen and elicit an immune response. This category includes DNA vaccines, RNA vaccines, and viral vector vaccines. They differ in the type of nucleic acid used and the method of delivery to host cells.

These various vaccine types are designed to stimulate the immune system and generate an immune response against specific pathogens, providing long-lasting protection against diseases. The choice of vaccine type depends on factors such as the nature of the pathogen, safety considerations, and the desired immune response.

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.

Active Vs Passive Immunity

Active Immunity	Passive Immunity
Produced actively by the host’s immune system	Received passively, without active host participation
Antibodies induced by infection or immunogens	Antibodies transferred directly into the host
Involves both cell-mediated and humoral immunity	Relies on pre-existing, readymade antibodies
Types: Natural – clinical or inapparent infection; Artificial – induced by vaccines	Types: Natural – transfer of maternal antibodies through the placenta; Artificial – injection of immunoglobulins
Immunity becomes effective after a lag period	Provides immediate immunity with no lag period
Durable, long-term, and effective protection	Transient, short-lived, and less effective
Immunological memory is present	No immunological memory
Booster effect observed on subsequent doses	Subsequent doses are less effective due to immune elimination
Can potentially lead to autoimmune disorders and allergic reactions	Can cause serum sickness
Not applicable in immunodeficient individuals	Applicable even in immunodeficient individuals
Used for prophylactic treatments	Used as a post-therapeutic remedy

FAQ

References

• 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, https://doi.org/10.1093/occmed/kqm110
• https://www.health.com/condition/infectious-diseases/active-vs-passive-immunity
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