CD40LG and shigellosis: This will include evaluation of anti-LPS serum IgA, IgM, or IgG subclass responses and of antibody avidity, and of anti-proteins (including non-vaccine antigens) responses which could help to elucidate the differences in priming between natural exposure and vaccination and possibly explain the apparent trend for an increased incidence of various clinical markers of shigellosis (including those for more severe shigellosis) post-challenge, in vaccinated individuals.