Active immunization can be achieved by natural infection with a microorganism, or it can be acquired artificially by admin-istration of a vaccine. In active immunization, as the name implies, the immune system plays an active role. Proliferation of antigen-reactive T and B cells results in the formation of mem-ory cells. Active immunization with various types of vaccines has played an important role in the reduction of deaths from infectious diseases, especially among children. Vaccines may be (a) live attenuated, (b) killed, or (c) in the form of toxoids.
These vaccines, as the name suggests, contain live attenuated organisms that have lost their pathogenicity but have anti-genicity. The attenuated organisms are the suspensions of live organisms with reduced virulence. These organisms multiply in the body and thereby provide a continuous antigenic stimulus, resulting in production of protective antibodies.
Bacille Calmette–Guérin (BCG); smallpox vaccine; oral polio vaccine (OPV); mumps, measles, and rubella (MMR) vac-cine; and yellow fever vaccine are some of the examples of live vaccines. These live vaccines are usually contraindicated for use in immunocompromised patients, such as patients with HIV (human immunodeficiency virus), leukemia, malignancies, etc.
These vaccines contain killed pathogens, hence do not replicate in body. At least three doses of killed vaccine followed by a booster dose are essential to confer protective immunity. Typhoid, cholera, pertussis, pneumococcal, rabies, hepatitis B, and influenza vaccines are the examples of killed attenuated vaccines.
Toxoids are modified toxins, which have retained their antigenicity but have lost their toxicity.
Adsorption of toxoid to a mineral carrier, such as aluminum hydroxide or aluminum phosphate enhances antigenicity of toxoids. This is because these adsorbed toxoids remain longer in a depot after injection and continue to stimulate immune system of host for a longer period of time. These are usually prepared by treating toxins with formalin.
Subunit vaccines are specialized vaccines that are prepared by purifying the fragments of major immunogenic components of microorganisms and are produced by recombinant DNA technology (hepatitis B subunit vaccine).