Immunity, along with the ability to limit pathogen growth to prevent significant body tissue damage, can be gained either by (1) actively developing an immune response within the individual after exposure to a pathogen or after getting vaccinated or (2) passively transferring immune components from an immune individual to one who is nonimmune. Both these forms of immunity can be found naturally and in medical practices.

Active Immunity

Active immunity refers to the resistance one develops against pathogens as a result of the adaptive immune response. Active immunity is acquired naturally during a bacterial or viral infection. This process enables the body to recognize and fight off the pathogen more effectively if it encounters it again in the future. Active natural immunity is typically acquired through infection and recovery from the disease, providing long-term protection. Artificially acquired active immunity is obtained through vaccines—these are killed or weakened pathogens or their components, or conjugated toxins, that reduce the effects of the secondary exposure. They facilitate the development of immunological memory (a weak primary immune response) without experiencing acute infection. The introduction of vaccines was a significant medical breakthrough in the 20th century, leading to the eradication of smallpox and control of numerous infectious diseases, including polio, measles, and pertussis.

Passive Immunity

Conversely, passive immunity results from transmitting antibodies to an individual without requiring them to generate their active immune response. Naturally acquired passive immunity can be observed during fetal development when IgG is transferred from the mother to the fetus via the placenta, which protects the fetus and the newborn during its initial months. Additionally, a newborn gains IgA antibodies from breast milk, benefiting from immunological memory based on the pathogens exposed to the nursing individual.

In medical scenarios, artificially acquired passive immunity typically involves immunoglobulin injections derived from humans or animals that have previously been exposed to a specific pathogen. This treatment provides a quick way to temporarily protect an individual who may have been exposed to a pathogen or toxin. The drawback to both forms of passive immunity is the absence of the development of immunological memory. After antibody transfer, protection is only temporary and lasts only until the antibodies degrade.