CD4 T cell division in response to the three pools of the relatively conserved peptides determined by flow cytometry using PKH26 dye. The results are expressed as stimulation indices; i.e. the mean number of dividing cells in triplicate wells containing an antigen divided by the mean number of dividing cells in triplicate wells without antigen. All responses with stimulation indices greater than or equal to 2 were considered as positive (i.e. those with values above the horizontal line shown on each of the three graphs). The prevalence of responders in each group is given below the graphs.