PASSIVE IMMUNITY: NATURAL, ARTIFICIAL, HISTORICAL COUNT - MEDICINE - 2025

The passive immunity is a form of acquired immunity that does not involve the immune response of the recipient (host). It consists of the transfer of antibodies previously produced by an organism exposed to an antigen to another organism that has not been in contact with said antigen.

Immunity is defined as a state of natural or acquired resistance against some infectious agent or some toxin or poison. An antigen is a substance recognized as foreign or toxic that binds in the body to a specific antibody and, as a consequence, may or may not trigger an immune response.

Photograph of a 30-week fetus which has received from its mother, through the placenta, the antibodies necessary to survive during the first months of life (Source: Ivon19, via Wikimedia Commons)

Passive immunity can be acquired naturally or artificially. 1) The natural form occurs when, through the placenta, the mother transmits the antibodies to the fetus or through the mother's colostrum to the newborn. 2) The artificial way is when specific antibodies against some pathogen, toxin or foreign substance are administered to an individual who is not immune.

Artificially acquired passive immunity was the form of treatment for infectious diseases before the age of antibiotics.

Currently it is used when immediate protection is required, for the treatment of diseases that cause immunodeficiency, to treat some poisonings and in emergencies to treat rabies, tetanus or snake bites.

Examples are human or animal blood plasma, human immunoglobulin, monoclonal antibodies, and antivenom. Passive immunity does not generate memory and is short-lived.

Brief historical account

Emil von Behring and Shibasaburo Kitasato, in 1890, reported that the injection of diphtheria toxins or tetanus bacilli toxin into animals stimulated in their organisms the production of substances that neutralize said toxins.

Furthermore, the blood serum of these animals that had developed diphtheria or tetanus antitoxin, when injected into other healthy animals, conferred immunity without having been in contact with the causative agents and even cured those that were already sick.

These authors concluded that immunity was conferred by substances called antitoxins present in the blood and that these substances were highly specific to protect only against one particular disease and not another.

Around the same time, other researchers showed that acquired immunity could be transmitted from the mother to the fetus through the circulation and to the newborn through colostrum (the mother's milk of the first days); it was later that a differentiation was made between passive and active immunity.

Natural passive immunity

This type of passive immunity is transmitted by the mother to the fetus or newborn. What are transmitted are antibodies that give the fetus or newborn humoral immunity (which is what has to do with the production of antibodies).

The antibodies that the mother transmits to the fetus through the placenta or through colostrum to the newborn are immunoglobulins.

Immunoglobulins, together with major histocompatibility complex molecules and T-cell antigen receptors, constitute the three types of molecules that the immune system uses to recognize specific antigens.

Immunoglobulins (Ig) are glycoproteins belonging to the group of plasma gamma globulins produced by B lymphocytes. There are several classes of antibodies called isotypes. Among these are: IgA, IgD, IgE, IgG and IgM.

Maternal IgG and IgA

Newborns do not have the ability to organize an effective immune response against microorganisms. However, the antibodies transmitted by the mother give the fetus and the newborn a protective action.

Through the placenta, the mother transmits IgG to the fetus and, through the milk, the newborn receives IgA, which works by neutralizing microorganisms that can colonize the intestine. Maternal IgG is also in milk and is transported from the intestine to the newborn's circulatory system.

The passage of maternal IgG through the intestine occurs through an intestinal receptor that the newborn has, which is an IgG receptor called the neonatal FcRN receptor. This receptor also has protection functions of IgG against cell degradation.

IgG antibodies are the most important immunoglobulins, both inside and outside the vessels. They act against infectious agents that spread through the blood. They facilitate phagocytosis of small particles and can activate the complement system, thus increasing phagocytic activity.

IgA is quite abundant and is produced in large quantities by the lymphoid tissue of the intestine, in the genitourinary tract and in the respiratory tract.

Its function is to prevent the entry of harmful organisms by forming non-absorbable complexes in the external secretion systems of man. These are saliva, tears, and bronchial, nasal, intestinal, and breast secretions.

Human milk contains IgA antibodies against a variety of infectious agents such as Vibrio cholerae, Campylobacter jejuni, Escherichia coli, Shigella, Salmonella, and some Rotaviruses. This protects the newborn from diarrheal diseases caused by these microorganisms.

Artificial passive immunity

In this immunity, specific antibodies are supplied against a certain antigen. The host that receives these antibodies develops immunity rapidly, in a matter of hours. Since these antibodies are not the result of exposure to the antigen, no memory is stored.

This immunity lasts only a few weeks, as the immunoglobulins injected with the serum have a half-life after which they are metabolized. Artificial passive immunity can also be acquired by receiving T cells from another organism.

Illustration referring to a blood transfusion (Source: Fæ, via Wikimedia Commons)

In addition to the speed with which immunity is obtained with artificial administration of antibodies, unlike vaccination, the protection obtained is independent of the immune status of the host.

For this reason, it is useful against bioterrorism and as the therapy of choice in endemic areas where vaccination has a poor response. It is also useful in hospitalized, malnourished or immunocompromised patients or in those patients in whom vaccination is contraindicated.

The type of antibody to be used for therapy will depend on the route of administration, the microorganism to be fought, and various economic factors.

For example, certain classes of IgA are more resistant to proteolytic degradation than others and have the advantage of being able to be administered orally, while others must be administered parenterally.

References

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4. Keller, MA, & Stiehm, ER (2000). Passive immunity in prevention and treatment of infectious diseases. Clinical microbiology reviews, 13 (4), 602-614.
5. Marcotte, H., & Hammarström, L. (2015). Passive Immunization: Toward Magic Bullets. In Mucosal immunology (pp. 1403-1434). Academic Press.
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