In most animals the combination of innate resistance and stimulation of adaptive responses by antigen is adequate to cope with common infections (otherwise the species would not survive!). However, the immune system does have its shortcomings, and some of these can be overcome by artificial means. Indeed, the introduction of vaccines has probably saved more lives than any other medical intervention to date. But there are still no effective vaccines against many of the world’s most common infectious diseases, including HIV, tuberculosis and malaria.
Most effective vaccines need to stimulate both innate and adaptive immunity. Adaptive immune responses suffer from their initial slowness, so that high levels of antibody may arrive too late to prevent death or disability (e.g. tetanus, polio) even though surviving patients are resistant to reinfection. Specific immunization overcomes this problem by ensuring there is a high level of immunity before exposure. This may be active (top right), in which antigen is used to safely generate immunological memory, aided in some cases by the boosting power of special non-specific stimulants or adjuvants (top left), or passive, in which preformed antibody is injected, with more rapid but short-lived effect.
Immunotherapy, as distinct from vaccination, refers to stimulating immune responses to cure, rather than prevent, disease. In general, conventional vaccines are ineffective when administered after exposure, although there are exceptions (rabies, chickenpox vaccination for prevention of shingles).
Finally, when some component of the immune system is deficient (see Fig. 33), efforts can be made to correct this by replacement of hormones, enzymes, cytokines, cells or organs.
Despite 200 years of cumulative success, there is a growing irrational fear of vaccination in the industrialized world and a corresponding rise in cases of dangerous illnesses such as measles and polio. Continued efforts at educating the public are required to ensure society benefits fully from the benefits of universal vaccination.
Adjuvants
are materials that increase the response to an antigen given at the same time. One way in which many adjuvants work is by creating a slow-release depot of antigen, thus prolonging the time for which the immune system remains in contact with antigen. In addition, they contain substances that activate macrophages and dendritic cells and via this pathway also increase antigen presentation (see Fig. 18). The most powerful adjuvants (e.g. Freund’s complete, which contains extracts of Mycobacterium tuberculosis
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