cytokines such as tumor necrosis factor and evidence of complement activation both in peripheral blood and in cerebrospinal fluid. The specific epitopes and their origins, whether derived from the host or the infectious agent, or both, remain uncertain. Numerous anti-nerve antibodies that bind to various protein, glycoprotein, and glycolipid moieties have been described in patients with GBS, but none occur in more than a fraction of cases. Whereas the P2 myelin protein, which is specific for peripheral nerve, and selected peptide fragments of it are capable of inducing experimental allergic neuritis under appropriate conditions, the P2 myelin protein does not appear to play a role in GBS.
A large number of studies have examined the incidence of GBS in many parts of the world. These show a relatively uniform occurrence of about 1 to 2 cases per 100,000 population per year in all populations examined, mainly occurring throughout the year and in all age groups. Epidemic outbreaks have been rare and imperfectly documented. The swine flu incident of 1976-1977 is, perhaps, the most completely described outbreak (Langmuir et al., 1984; Safranek et al., 1991; Schonberger et al., 1979). The unusual circumstances of the swine flu incident should be noted. Over 40 million people were vaccinated in a period of a few weeks, an unprecedented mass vaccination program. It is likely that the excess cases of GBS might not have been detected if the numbers of people vaccinated had not been so large. A seasonal incidence of clinical GBS occurs annually in children and young adults in the northern part of the People's Republic of China (McKhann et al., 1991, in press), although the electrodiagnostic and pathologic features in these cases indicate a severe axonal lesion and not the usual demyelinating process with inflammation of the delayed hypersensitivity type.
A persistent problem has been the uncertainty about the expected incidence of GBS unrelated to vaccination in the cohort under 5 years of age. There is reasonably good information to suggest that the overall incidence of GBS for all ages is about 1 case per 1,000,000 population per month. Many authorities have suggested that the incidence of GBS in the pediatric age group (0-16 years of age) is lower than that in adults. For a number of years, the literature has provided data indicating that the incidence of GBS in the cohort under the age of 5 years is higher than that in children older than that (Beghi et al., 1985b; Coe, 1989; Soffer et al., 1978) and one study found a high incidence in preschool children of 5.4 cases per 100,000 per year (Kibel et al., 1983). Other observers have found a lower incidence of GBS in children, with the incidence distributed evenly from infancy to the teenage years (Hurwitz et al., 1981; Rantala et al., 1991; Uhari et al., 1989).