Understanding the principle and differences between passive and active immunization is important when considering the underlying mechanisms of many clinical applications of immunology. There are developmental concepts, pathological disorders, and treatments that depend on the distinct traits of these types of immunity.

Passive Immunization is the process by which an individual receives immunity to a particular antigen by getting a donation of antibodies from an outside source. A natural setting in which is occurs is in fetal development. During development IgG antibodies cross from the mother into the child's blood supply, giving them the same immunity as their mother for several weeks after birth. More on the process of Maternal Passive Immunity can be found in this NCBI article. Passive Immunity can also be used in treatment of certain acute illnesses. For example, antivenoms work by injecting the affected person with a dose of antibodies produces from a second source that has specific affinity for a particular toxin. In both cases, Passive Immunity is a temporary solution and the resistance it confers only persists until the antibodies are broken down. After that time, the patient is once again susceptible to the antigen.

Active Immunization is the process by which an individual produces its own immune defense in response to a pathogen. This type of immunity leads to the formation of memory cells that confer immunity for long periods of time (usually for life). Active Immunity is the natural process that keeps us safe form pathogens that we encounter in everyday life and it is the principle upon which vaccination is based. When an inactive form of a pathogen is presented to a person in the form of a vaccine, the immune system responds to it and begins producing memory cells that await a second infection by the pathogen. When the active form of the microbe makes its way into the patient, the immune system already is already producing antibodies and other immune responses to curb the infection before it starts. Specific details about the mechanism of vaccinations can be found in this NCBI article.