|MadSci Network: Immunology|
Very interesting question. (I hope the snow has stopped in the Chicago area.) The Organisms Side First, some pathogens continually change or mutate their protective capsules or coat. This is the case for influenza, where there are two major serotypes A and B. The A tends to be more severe and we usually include two A substrains in each year`s vaccine and only one B strain. Of course, the virus changes its coat proteins e.g., the the hemagglutinin each year and in so doing can fool the immune system. In this case you may have nice immunity to last year's strain, including antibodies and cytotoxic T cells but the mutated strain can change its coat to avoid being neutralized (eliminated). What we need is a broad based vaccine that is given by the nasal route and which will induce a long-lasting mucosal and systemic immune response that will protect you year in and year out. By the way, just such a vaccine is in final phase 3 trials and will be available for the public soon. Why are booster shots necessary for vaccines like cholera or tetanus ? Let`s use tetanus as the example since we really do not have a suitable cholera vaccine. Infants receive their DPT shots, which includes tetanus toxoid at 2, 4 and six months of age. The reason for this multiple injection is two-fold. First, the babies will have various levels of antibodies to tetanus toxin from their mothers. The titer will vary and if the titer is high the first immunization of the baby will not work. Instead of immunizing, the baby's anti- tetanus toxin antibodies will simply bind to the vaccine and eliminate it. Thus, one keeps immunizing until the baby starts to make his/her own immune response. The second point is also interesting. Tetanus toxoid vaccine is excellent; however your immune response will wane without boosting. You still have some memory T and B cells but you cannot respond fast enough to make anti-toxin antibodies to protect you if you encounter the organism Clostridium tetani that produces this powerful exotoxin. The trick is that you need to have a certain level of circulating antibodies to the exotoxins to protect you. If you do not, the organism can produce enough toxin to kill you before your memory T and B cells can expand and make a new antibody response.
Try the links in the MadSci Library for more information on Immunology.