After a natural infection the human immune system in most cases launches an immunological response to the particular pathogen. After recovery from the disease, the immunological response indeed protects the affected individual from that disease, in the ideal case forever. This phenomenon is called immunity and is due to the presence of circulating antibodies, activated cytotoxic cells and memory cells. Memory cells become active when the same type of antigenic material enters the body on a later occasion. Unlike the primary response after the first infection, the response after repeated infection is very fast and usually sufficiently strong to prevent reoccurrence of the disease.
The principle of vaccination is mimicking an infection in such a way that the natural specific defense mechanism of the host against the pathogen will be activated, but the host will remain free of thedisease that normally results from a natural infection. This is effectuated by administration of antigenic components that (1) consist of, (2) are derived from, or (3) are related to the pathogen. The success of vaccination relies on the induction of a long-lasting immunological memory. Vaccination is also referred to as active immunization, because the host’s immune system is activated to respond to the “infection” through humoral and cellular immune responses, resulting in adaptive immunity against the particular pathogen. The immune response is generally highly specific: it discriminates not only between pathogen species, but often also between different strains within one species (e.g., strains of meningococci, poliovirus, influenza virus). Albeit sometimes a hurdle for vaccine developers, this high specificity of the immune system allows an almost perfect balance between response to foreign antigens and tolerance with respect to self-antigens. Apart from active immunization, administration of specific antibodies can be utilized for short-lived immunological protec-tion of the host. This is termed passive immunization (Fig. 2).
Traditionally, active immunization has mainly served to prevent infectious diseases, whereas passive immunization has been applied for both prevention and therapy of infectious diseases. Through recent developments new potential applications of vaccines for active immunization have emerged, such as the prevention of other diseases than infectious diseases (e.g., cancer) and for the treatment of substance abuse (e.g., nicotine addiction). Such vaccines are referred to as therapeutic vaccines. The difference between passive and active immunization for preventive and therapeutic applications is outlined in Figure 2. Since antibody preparations for passive immunization do not fall under the strict definition of a vaccine, they are not discussed here.