Active immunization is the more familiar type of immunization that people think of when they think about how preventive vaccines work. It’s the process of stimulating the immune system to produce antibodies against a specific pathogen. There are two ways to stimulate active immunity. One is natural immunity, acquired through exposure and infection with the organism, with or without disease (actual symptoms).
The other is vaccine-induced immunity, in which a killed or weakened form of the organism or an antigen from the organism, such as protein, induces the immune system to create antibodies. If it’s an antigen such as a protein, it may be a protein introduced by the vaccine, such as with conjugate vaccines, or a protein the body itself has created following instructions in the vaccines, as with messenger RNA (mRNA) or DNA vaccines.
Active immunization often involves more than just stimulating the body to create the antibodies. A vaccine, or exposure to the actual pathogen, also activates B cells (which produce antibodies) and T cells (which destroy infected cells and direct other immune cells to attack pathogens or infected cells), so active immunization stimulates B cell and T cell memory as well.
Some active immunization is lifelong, such as that induced by the measles vaccine, while other times the antibodies and immune cell memory induced by active immunization wane over time.