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Immunity is the body’s ability to resist infection by certain pathogens. Some immunity in the body is natural (or innate) and is provided by genetics and through the mother’s immune system during gestation. It provides some protection from disease as the child develops its own acquired immunity.

Most immunity is acquired, which means the body produces it as a result of environmental exposure to antigens. As a patient is repeatedly exposed to antigens, acquired immunity is strengthened. Each time the immune system encounters an antigen, memory cells (made from B cells and T cells) are stored. The next time the same antigen is encountered, the body is able to launch a faster immune system response to kill the pathogen.

Immunization is the process of providing acquired immunity to a patient. It is administered through a vaccine, or an inactivated amount of a particular disease-causing agent. Vaccines can be attenuated (weakened) live viruses, killed pathogens, toxoid (made from toxins produced by bacteria) or biosynthetic (a combination of natural and synthetic substances). Through vaccines, an antigen is introduced into the body so the immune system can develop protective antibodies without risk of exposure to the actual pathogen that causes disease. Some diseases for which vaccines have been developed include smallpox, measles, whooping cough, and tetanus.

Vaccines are usually injected, although some can be inhaled through a nasal spray or ingested through the mouth. Many vaccines provide a lifetime of protection against disease. Others need periodic updating (boosters). The Centers for Disease Control and Prevention (CDC) has schedules of the vaccines it recommends for infants (beginning at 2 months of age), children, adolescents and adults.

Another example of deliberate exposure to an antigen is the use of antiserum (immune serum). Given by injection, this provides antibodies to a patient for temporary immunity to a particular microorganism. Some serums are used prior to potential disease exposure, such as those administered to travelers before they visit places where tropical diseases may occur. Other serums are given following exposure to a disease or toxin, such as rabies or snake bite venom.

The other main source for acquired immunity is breastfeeding. Breast milk contains antibodies and can be an infant’s primary source of acquired immunity.