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Why Not Vaccinate Against All Infectious Diseases?

Some early immunologists dreamed of developing a broad complement of vaccines that would make people resistant to all infectious diseases. Yet today we still cannot immunize against many important diseases-gonorrhea, syphilis, AIDS and even the common cold continue to strike millions of victims each year. The lack of suitable vaccines against many diseases is due partially to the nature of these pathogens and partially to characteristics of the immune system. The following list describes some of the barriers to immunizations.
·         Some pathogens such as Neisseria gonorrhoeae are so nonimmunogenic that they fail to trigger protective levels of immunity. Extensive efforts to produce immunogenic vaccines against gonorrhea have as of 1994, yet to yield a promising product.
·         Infections localized on the skin or mucous membranes are not affected by circulating antibodies and are generally controlled by secretory lgA. Unfortunately immunologic recall is less efficient with these secretory immunoglobulins than with circulating antibodies. Thus uncomplicated gonorrhea and other diseases restricted to the urogenital mucosa stimulate poor natural immunity during infection and are less likely to be controlled by vaccines. In addition Neisseria gonorrhoeae produces a protease that protects the pathogen by enzymatically destroying lgA.
·         Some pathogens suppress immunologic responses during natural infection. Unfortunately vaccines prepared against these diseases may similarly depress immunity rather than enhancing it.
·         Some viruses become sequestered within host cells where they are protected from the body’s immune elements. Herpensviruses, for example, persist in nerve cells following disappearance of the epithelial lesions. Lesions may recur when the virus migrates within infected neurons directly to the skin even in the presence of specific circulating antibodies.
·         Vaccine production usually requires laboratory cultivation of the pathogen to provide a source of antigen. Unfortunately, some pathogens, notably Treponema pallidum (syphilis), cannot be readily grown in vitro.
·         The common cold viruses although extremely antigenic are so numerous it would be virtually impossible to develop vaccines against them all. More than 100 antigenically distinct cold viruses would have to be included in a complete vaccine. Such a vaccine would not only be impractical but would be of little value because poor immunity is induced by simultaneous administration of so many antigens.
·         Some pathogens periodically alter their surface antigens. The new antigens are not recognized by immune cells sensitized to the previous determinants. Influenza is the most extensively documented example of a pathogen’s tendency toward antigenic shifts. Although influenza vaccines are available, they are type-specific. A new vaccine must be developed whenever a new antigenically distinct strain appears. Development of a vaccine against AIDS has also been hampered by the ability of the human immunodeficiency virus to change antigenic specificity.
·         Vaccines are most successful against disease controlled by humoral immunity. Few vaccines are available against diseases that are controlled primarily by CMI.

·         Some pathogens immunologically camouflage themselves by coating themselves with host blood proteins. In this way the surface antigens of plasmodia (malaria) and trypanosomes are hidden from immune effectors. Vaccination against these diseases may trigger the formation of antibodies and lymphocytes that cannot locate their camouflaged antigens.
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