Immunity can be defined as the ability of a multicellular organism’s body to protect itself 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 defense 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 to 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.
Image Source: BioNinja.
Active Immunity vs Passive Immunity Table Form
|S.N.||Characteristics||Active Immunity||Passive Immunity|
|1.||Definition||The protective immunity in which the individual’s own immune system is stimulated to produce antibodies and lymphocytes.||The immunity in which a person receives antibodies or lymphocytes that have been produced by another individual’s immune system.|
|2.||Exposure to Antigen||Requires exposure to a pathogen or to the antigen of a pathogen.||Does not require exposure to an infectious agent or its antigen.|
|3.||Immune system involvement||The immune system of the individual is actively involved in the process.||The immune system of the individual is not actively involved but rather passive.|
|4.||Natural acquirement||Arise naturally when an individual is exposed to an antigen or pathogen (clinical infection).||Arise naturally when a fetus receives antibodies from the mother across the placenta or when a breastfeeding infant ingests antibodies in the mother’s milk.|
|5.||Artificial acquirement||Conferred artificially by means of vaccines.||Conferred artificially by administration of preformed antibodies.|
|6.||Immunity type||Involves both humoral and cell-mediated immunity.||The immunity is conferred only by readymade antibodies.|
|7.||Components||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).||No immune cells are involved as the antibody is preformed.|
|8.||Antibody production||Involves antibody production which is induced by infection or immunogen.||No antibody is produced but directly transferred.|
|9.||Memory cell formation||Active immunity results in the formation of long-lasting memory cells.||Memory immune cells are not formed.|
|10.||Secondary response||The first exposure leads to a primary response and in case of subsequent exposure to the same pathogen later, a much faster and stronger secondary response is established.||Absence of a secondary response.|
|11.||Durability||The protection offered is long-lived.||The protection is only transient.|
|12.||Response time||The protective response takes time to establish as a lag period is present.||No lag period hence the protection is instant.|
|13.||Reactivation||Reactivated by the recurrence of infection or by revaccination.||Frequent re-administration is needed for renewed protection.|
|14.||Booster effect||Subsequent doses with antigens cause booster effects.||Subsequent doses are less effective due to immune elimination.|
|15.||Suitability||Active immunity is not suitable for the protection of immuno-compromised or immuno-deficient individuals.||Passive immunity is useful in cases of immuno-compromised, immuno-deficient, or severe combined immunodeficiency.|
|16.||Use||Very effective for prophylaxis of diseases.||Artificial passive immunity is effective as a post-exposure remedy.|
|17.||Effectiveness of Protection||Provides effective protection.||Protection rendered is less effective and may not be complete.|
|18.||Adverse effect||It can be implicated in autoimmune diseases and allergies, but generally does not have side effects.||A condition called serum sickness can result from exposure to antisera.|
|19.||Examples||Natural – Producing antibodies in response to exposure to a pathogenic infection such as measles or cold. |
Artificial – Producing antibodies in response to the controlled exposure to an attenuated pathogen (i.e. vaccination).
|Natural – Receiving antibodies from another organism (e.g. to the fetus via the colostrum or a newborn via breast milk).|
Artificial – Receiving manufactured antibodies via external delivery (e.g blood transfusions of monoclonal antibodies).
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, that 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).
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.
- The College of Physicians of Philadelphia (2018, January 10). Passive Immunization. Retrieved from https://www.historyofvaccines.org/content/articles/passive-immunization
- Encyclopaedia Britannica (2017, November 22). Immunization. Retrieved from https://www.britannica.com/science/immunization.
- Lydyard, P.M., Whelan,A.,& Fanger,M.W. (2005).Immunology (2 ed.).London: BIOS Scientific Publishers.