Various antibodies directed at nerve cells have been reported in Guillain–Barré syndrome. In the axonal subtype, these antibodies have been shown to bind to gangliosides , a group of substances found in peripheral nerves. A ganglioside is a molecule consisting of ceramide bound to a small group of hexose -type sugars and containing various numbers of N -acetylneuraminic acid groups. The key four gangliosides against which antibodies have been described are GM1 , GD1a, GT1a, and GQ1b, with different anti-ganglioside antibodies being associated with particular features; for instance, GQ1b antibodies have been linked with Miller Fisher variant GBS and related forms including Bickerstaff encephalitis.  The production of these antibodies after an infection is probably the result of molecular mimicry , where the immune system is reacting to microbial substances but the resultant antibodies also react with substances occurring naturally in the body.   After a Campylobacter infection, the body produces antibodies of the IgA class; only a small proportion of people also produce IgG antibodies against bacterial substance cell wall substances (. lipooligosaccharides ) that crossreact with human nerve cell gangliosides. It is not currently known how this process escapes central tolerance to gangliosides, which is meant to suppress the production of antibodies against the body's own substances.  Not all antiganglioside antibodies cause disease, and it has recently been suggested that some antibodies bind to more than one type of epitope simultaneously (heterodimeric binding) and that this determines the response. Furthermore, the development of pathogenic antibodies may depend on the presence of other strains of bacteria in the bowel.