- Immunity can be defined as the ability of a multicellular organism’s body to protect themselves from foreign agents and infectious agents, by being able to destroy them to prevent further body infections.
- Immunity to infectious microorganisms can be achieved by active or passive immunization. In each case, immunity can be acquired either by natural processes (usually by transfer from mother to fetus or by the previous infection by the organism) or by artificial means such as injection of antibodies or vaccines.
- Immunity allows the body to fight off infections. And therefore, immunity is composed of two major lines of defenses namely: Innate immunity and adaptive immunity.
- Innate Immunity is the first line of defense in the human body. It is a natural (born with) or a genetic immune defense mechanism. Innate immunity can be either external defense or internal defense. External defense works in protecting the body from exposure to pathogens and it includes skin, tears, and stomach acid. While internal defense otherwise known as the second line of defense includes fever and inflammation, and the mechanisms of phagocytes.
- Adaptive immunity, also known as acquired immunity, is the third line of defense, which is activated by innate immunity. This type of immunity and the immune responses elicited are acquired by experience only, which means that the immune response is induced by the host’s response to a foreign antigen (pathogen) or by the transfer of antibodies or lymphocytes that are specific for the pathogen. It involves the defense by immune cells and other activated agents such as cytokines. This type of immunity protects the body against specific pathogens.
- Adaptive immunity can be classified into active immunity and passive immunity.
- Active immunity develops due to the production of antibodies in the body. Active immunity is attained by exposure to a pathogen leading to the production of antibodies in the body.
- Passive immunity is developed by antibodies that are produced outside and then introduced into the body. Passive immunity is acquired when antibodies are introduced into the body from an external source (usually through vaccines) providing a quick response to the infection.
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- The immunity induced by exposure to a foreign antigen is called active immunity. Active immunity confers a form of resistance against a foreign antigen when encountered by an individual. Therefore active immunity can be gained through
- clinical or subclinical infection – this is previous exposure to an antigen which might have caused an infection
- immunization with live or killed infectious agents or their antigens via vaccination
- exposure to microbial products, such as toxins and toxoids.
- These forms of exposures to an antigen, the host immune system stimulates an immune response against the foreign antigens, and these immune responses can be by activation of antibodies by the B-cells and T-cells activation (T-helper and T-cytotoxic/Cytotoxic T-cells).
- After the activation of these immune mechanisms, and a latent period of infection and body immune responses, then the active immunity is developed. The host immunity is then geared up to act against the foreign pathogen/antigen. This means that immune responses are normally slow on the onset (primary response), but once the active immunity is developed, it forms a long-lasting immunity. This is the major advantage of active immunity.
Characteristics of Active Immunity
- Active immunity requires exposure to a pathogen or to the antigen of a pathogen.
- Exposure to the antigen leads to the production of antibodies. These antibodies essentially mark a cell for destruction by lymphocytes.
- Cells involved in active immunity are 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).
- There is a delay between exposure to the antigen and acquiring immunity. The first exposure leads to a primary response. If exposed to the antigen again later, the response the immune system elicits is much faster and stronger. This is called a secondary response.
- Active immunity is a long-term type of immunity that can last a lifetime.
- Some of the known side effects of active immunity are resultant autoimmune diseases and allergic responses.
Types of Active Immunity
A. Natural active immunity
- This is the immunity that is acquired when an individual is exposed clinically or subclinically through infection.
- This means that the individual is exposed to the live pathogen, develops the disease, and develops a long-term form of immunity as a result of the primary response.
- When a microbe penetrates the body’s barriers such as the skin, mucous membranes, or other primary defenses, it interacts with the immune system activating the B-cells to produce antibodies that help to fight the invading pathogen.
- The adaptive immune response generated against the pathogen takes days or weeks to develop but may be long-lasting, or even lifelong.
- In cases that involve wild infection, such as hepatitis A virus (HAV) and subsequent recovery, it gives rise to a natural active immune response usually leading to lifelong protection.
B. Artificial active immunity
- Now, in this case, an administration of a vaccine will generate an acquired artificial active immunity.
- For example, administration a the two doses of hepatitis A vaccine will generate an acquired active immune response which will lead to long-lasting or life-long protection against hepatitis A.
- This type of immunity is acquired through immunization via vaccination. Vaccination is a deliberate induction of an immune response by introducing an antigen (part of a disease-causing pathogen) into the body to activate the body in producing an immune response against the antigen, rendering a long-term memory response in case of another exposure to the antigen.
- Vaccination represents the single most effective manipulation of the immune system that scientists have developed.
- Immunizations are successful because they utilize the immune system’s natural specificity as well as its inducibility. The principle behind immunization is to introduce an antigen, derived from a disease-causing organism, that stimulates the immune system to develop protective immunity against that organism, but which does not itself cause the pathogenic effects of that organism.
- Therefore artificial active immunity is induced in individuals by vaccines.
- There is a wide range of vaccines available against many microbial pathogens. These may be live vaccines, killed vaccines, or vaccines containing bacterial products.
Mediators of active immunity
Active immunity is mediated by humoral immunity and cell-mediated immunity and their components. The principle function of these immune responses is to eliminate or kill different types of pathogens.
- Humoral immunity: This immunity offers the mediation of antibody functions, found in blood and the mucosal secretions. These antibodies are secreted by B-cells plasma cells, and they are able to recognize microbial antigens, neutralizing their infectivity, and target them for elimination by various effector mechanisms. Humoral immunity is the principal defense mechanism against extracellular microbes.
- Cell-mediated immunity: This immunity is mediated by both activated TH cells and CTLs. Cytokines secreted by TH cells activate various phagocytic cells, enabling them to phagocytose and kill microorganisms. This type of cell-mediated immune response is especially important against a host of bacterial and protozoal pathogens. CTLs play an important role in killing virus-infected cells and tumor cells. They act by killing altered self-cells.
- Passive immunity is more of a form of therapy where antibodies are introduced from outside the host. They can otherwise be known as ‘loaned’ antibodies.
- The loaned antibodies help prevent or fight certain infectious diseases, however, the protection offered by this form of immunity is short-lived, lasting a few weeks or months. It otherwise offered temporal protection.
- For example, emergency use of immune serum, containing antibodies against snake or scorpion venoms, is a common practice for treating bite victims. This form of immune protection that is transferred between individuals is called passive immunity because the individual receiving it did not make his or her own immune response against the pathogen.
- Newborn infants benefit from passive immunity by the presence of maternal antibodies in their circulation.
- Passive immunity may also be used as a preventive (prophylaxis) to boost the immune potential of those with compromised immunity or who anticipate future exposure to a particular microbe.
- While passive immunity can supply a quick solution, it is short-lived and limited, as the cells that produce these antibodies are not being transferred.
Characteristics of Passive Immunity
- Passive immunity is conferred from outside the body, so it doesn’t require exposure to an infectious agent or its antigen.
- There is no delay in the action of passive immunity which means the response to an infectious agent is immediate.
- Passive immunity is not as long-lasting as active immunity lasting a few days, or weeks or months.
- However, negatively, serum sickness can result from exposure to antisera.
Types of Passive Immunity
A. Natural Passive Immunity
- This occurs during pregnancy when antibodies passing from maternal blood into the fetal bloodstream.
- It is observed when IgG passes from the mother to the fetus (during pregnancy), forming a basis of prevention of neonatal tetanus in neonates by active immunization of pregnant mothers.
- This is achieved by administering the tetanus toxoid to pregnant mothers during pregnancy (first times – three doses) or during the last trimester for second-time mothers.
- This induces the production of high-level antibodies in the mother against the tetanus toxin, which is subsequently transmitted to the fetus, from the mother through the placenta.
- The antibodies then protect the neonate after birth against the risks of tetanus.
- Natural passive immunity is also observed by the passing of IgA from the mother to the newborn through breastfeeding.
B. Artificial Passive Immunity
- Artificial passive immunization is normally administered by injection.
- It is used if there has been a recent outbreak of a particular disease or as an emergency treatment for toxicity, such as tetanus.
- It is induced in an individual by administering preformed antibodies, in form of antiserum, raised against an infecting agent.
- The antibodies can be produced in animals, called “serum therapy,” although there is a high chance of anaphylactic shock because of immunity against animal serum itself. Therefore, humanized antibodies produced in vitro by cell culture are used instead, when and if available.
- Administration of these antisera makes large amounts of antibodies available in the recipient host to neutralize the action of toxins.
- The preformed antibodies against rabies and hepatitis A and B viruses, etc. given during the incubation period prevent replication of the virus, and hence alter the course of infection.
- Immediate availability of a large number of antibodies is the main advantage of passive immunity.
- However, the short lifespan of these antibodies and the possibility of hypersensitivity reaction, if antibodies prepared in other animal species are given to individuals who are hypersensitive to these animals globulins (e.g., serum sickness), are the two noted disadvantages of passive immunity.
Applications of Passive Immunity Today
- Patients today can be treated with antibodies when they are sick of diphtheria or cytomegalovirus.
- Antibody therapy can also be used as a preventive measure on exposure to pathogens in order to stop the development of an illness such as the Respiratory Syncytial Virus (RSV), measles, tetanus, hepatitis A and B, rabies, and Chickenpox.
- However, routine treatment with antibodies for these diseases is healthy, but it could benefit high-risk individuals especially those with immune deficiencies.
The Future of Passive Immunization
- Monoclonal antibody production technology is increasing, thus a pure, single type of antibodies from the single-parent cells that are targeted at a single site on a pathogen are produced. These antibodies have wide-ranging potential applications to infectious disease and other types of diseases.
- Currently, there is only one MAb treatment is commercially available for the prevention of an infectious disease. This is a MAb preparation for the prevention of severe disease caused by RSV in high-risk infants.
- Physicians are also increasingly using MAbs to combat noninfectious diseases, such as certain types of cancer, multiple sclerosis, rheumatoid arthritis, Crohn’s disease, and cardiovascular disease.
- Scientists are researching other new technologies for producing antibodies in the laboratory, such as recombinant systems using yeast cells or viruses and systems combining human cells and mouse cells, or human DNA and mouse DNA.
B. Bioterror threats
- Passive immunization can also play a major role in a bioterrorism attack (release of an infectious biological agent), as an emergency response. The advantage of using antibodies rather than vaccines to respond to a bioterror event is that antibodies provide immediate protection, whereas vaccine protection is not immediate and in some cases may depend on a booster dose given at a later date.
- Candidates for this potential application of passive immunization include botulinum toxin, tularemia, anthrax, and plague. For most of these targets, only animal studies have been conducted, and so the use of passive immunization in potential bioterror events is still in the experimental stages.
Advantages of Passive Immunization
- Passive immunization acts quicker, producing an immune response within hours or days of the administration, unlike vaccines, which take weeks or months to produce protective immunity.
- Passive immunization also can override a deficient immune system, which is very helpful to persons that do not respond to vaccine immunizations.
Disadvantages of Passive Immunization
- Antibodies are costly to produce, however, new technologies can help produce them in the laboratory especially antibodies to infectious diseases that must be harvested from the blood of thousands of donors or even obtained from the blood of immune animals like those produced to neutralize snake venoms.
- Antibodies from animals can cause serious allergic reactions to the recipient.
- Antibodies must be administered via intravenous injection, which can be a complicated procedure and time consuming that the injection of a vaccine.
- The immunity conferred by passive immunization is short-lived i.e it does not lead to the formation of long-lasting memory immune cells.
Combined Active and Passive Immunity
- Combined passive-active immunity is carried out by giving both preformed antibodies (antiserum) and a vaccine to provide immediate protection and long-term protection, respectively, against a disease. This approach is followed for the prevention of certain infectious conditions, namely, tetanus, rabies, and hepatitis B.
- Another scenario is, a person bitten by a rabid animal might receive rabies antibodies (passive immunization to create an immediate response) and rabies vaccine (active immunity to elicit a long-lasting response to this slowly reproducing virus).
Difference between Active Immunity and Passive Immunity
|S.N.||Active Immunity||Passive Immunity|
|1.||It is produced actively by the host’s immune system||No active host participation; received passively|
|2.||Antibodies induced by infection or by immunogens||Antibodies transferred directly into the host|
|3.||Active immunity often involves both the cell-mediated and humoral immunity||Passive immunity is due to readymade antibodies|
|4.||Types: Natural—clinical or inapparent infection;
Artificial— induced by vaccines
|Types: Natural—transfer of maternal antibodies through the placenta; Artificial—injection of immunoglobulins|
|5.||Immunity effective only after a lag period||Immediate immunity; no lag period|
|6.||Durable; long-term and it is an effective protection||Transient; short-lived and it is less effective|
|7.||Immunological memory present||No immunological memory|
|8.||Booster effect on subsequent dose||Subsequent dose less effective due to immune elimination|
|9.||A negative phase may occur||No negative phase|
|10.||Not applicable in immunodeficient||Applicable even in immunodeficient|
|11.||Used for prophylactic treatments||Used as a post-therapeutic remedy|
|12.||It is a possible cause of autoimmune disorders and allergic reactions||It can cause serum sickness|
The Debate on Active and Passive Immunity and COVID-19
- Research is underway on understanding how Active and Passive Immunity can benefit our immune systems against the novel coronavirus infection, COVID-19.
- It has been noted that people who contract the infection, develop antibodies within two weeks but their form of protection is not well understood yet, and still unclear if a person can contract a second SARS-CoV-2 infection.
- The ongoing studies are focusing on the neutralizing antibodies which bind directly to the viral proteins preventing the virus from infecting the cell. It should also be noted that, from studies, there are no circulating neutralizing antibodies found in blood, but potential B-cells producing neutralizing antibodies were present.
- This could mean that antibodies produced in active immunity do not last-long in fighting the viral infection, but studies are still ongoing to understand its role in COVID-19 infection.
- Passive immunity has however shown promise whereby the seriously ill patients are treated with a blood product called convalescent plasma. This is plasma that has been isolated from recovered patients, and it is given intravenously to seriously ill patients. But studies are still ongoing to understand the effectiveness of this form of therapy to COVID-19.
Other forms of Immunity
- The immunity at a particular site, generally at the site of invasion and multiplication of a pathogen, is referred to as local immunity.
- Local immunity is conferred by secretory IgA antibodies in various body secretions.
- These antibodies are produced locally by plasma cells present on mucosal surfaces or in secretory glands.
- Natural infection or attenuated live viral vaccines are given orally or intranasally induces local immunity at gut mucosa and nasal mucosa, respectively.
- Herd immunity refers to an overall level of immunity in a community, which means that when enough people in a community have been exposed to a pathogen, then it can not spread easily.
- With herd immunity, as more people become immune, the pathogen will have a smaller pool of people to infect and result in fewer outbreaks within the community.
- To eradicate an infectious disease sometimes it depends on the development of a high level of herd immunity against the pathogen. An epidemic of disease is likely to occur when herd immunity against that disease is very low indicating the presence of a larger number of susceptible people in the community.
- However, not all pathogens spread with the same efficiency, the community levels of immunity necessary to benefit from herd immunity vary.
- For example, because measles is one of the most contagious pathogens known, a community requires almost everyone to be immune in order to stop its transmission. But it is much more difficult for an individual to benefit from herd immunity to measles than from most other infectious agents. Therefore, vaccines have made it easier for society to reap the benefits of this type of protection.
- Herd Immunity and COVID-19 has been debated upon by researchers stating that it can be achieved through natural infection but will have serious consequences, and therefore, it is not a solution to immunity to COVID-19.
References and Sources
- Kuby Immunology 5th Edition
- Kuby Immunology 7th Edition
- Microbiology and Immunology 2nd Edition by Shubash Chandra Parija
- 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