Immunity can be acquired naturally or artificially. In either case, the host is exposed to an antigen (foreign protein), the antigen is recognized, and the host builds a complex immune response to neutralize the antigen.
Vaccines artificially expose the host to antigens which then elicit an immune response. There are two types of vaccines: killed vaccines and modified live vaccines. Killed vaccines are composed of agent antigens but not living agent. Modified live vaccines are composed of non-virulent, living strains of the agent. For example, polio vaccine comes in both killed (Salk vaccine) and the modified-live (Sabin vaccine) forms. The Salk vaccine, developed in 1955, is made by cultivating three strains of the virus in monkey tissue. The virus is then separated from the tissue, stored for a week, and killed with formaldehyde. This killed vaccine is given by injection and requires 4 inoculations. The modified-live Sabin vaccine is an attenuated strain of living agent. It is thus capable of causing infection, but not disease. The Sabin vaccine is given orally and is the primary type of polio vaccine in use today since it is easier to administer and elicits a stronger response than the Salk vaccine (partially because viral multiplication creates a larger stimulus for response). The Salk vaccine is now mostly of historic interest.
Toxoids are harmless derivatives of microbiologic toxins that simulate an active immune response to toxins released by pathogens and other poisonous sources. For example, the tetanus toxoid immunizes the individual against the poison produced by the bacteria Clostridium tetani. C. tetani usually enters the body through a puncture, cut, or open wound and then releases a toxin that affects the motor nerves, which stimulate the muscles. Regular vaccination with the toxoid is needed to ensure immunity and thus prevent the painful spasms of muscles, including "locking" of the jaw so that the mouth cannot open (lockjaw) and death that otherwise might results.
B. Gerstman 5/20/03