Immunization occurs as the result of the penetration of an immunogen in an organism, leading to the stimulation of the immune system and the production of antibodies and/or T-cell effectors that specifically recognize the initiating antigen. Immunization may occur naturally, upon penetration of a pathogen, bacteria, virus, or parasite, and requires several days before becoming fully effective fighting the invader. This may be too slow to ensure efficient protection, and this led to the development of systematic vaccinations against major pathogens. The word vaccination originates from the first vaccine that was directed against smallpox, using the cross-reactive vaccinia virus. Used on a worldwide basis, it led to the complete eradication of this severe disease, and thus is no longer in use. Major vaccines used in humans include tetanus, diphtheria, measles, polio, mumps, rubella, influenza, rabies, hepatitis B, and a few others. Vaccines are most frequently administered by injection, but sometimes orally, as in the case of the live attenuated poliovirus vaccine. A vaccine may be used as the full pathogen after it has been inactivated, or as an attenuated genetic variant. One also can use a purified molecule that has been isolated from the pathogen or genetically engineered.
For laboratory purposes, immunization generally aims at producing antibodies for immunochemical analysis. Conventional polyspecific antibodies are generally prepared in mice, guinea pigs, rabbits, or goats. There is no standard immunization procedure, but only some general guide lines that define basic conditions for good immunogenicity. Besides using an appropriate dose, the antigen might have to be conjugated to a carrier protein. Use of adjuvant, most frequently the complete Freund’s adjuvant, is admixed with the immunogen. Finally, repeated injections will increase both the affinity and the titer of the antibodies generated.
