only blanks the average cpm values obtained for the blanks were subtracted from those obtained for the peptides the values thus obtained were added up to yield what was considered 100 stimulation by dtxspecific peptides the average cpm obtained for each irs minus blank were added and the fraction of the total response which this sum represented was calculated preferential recognition of certain epitopes might be due to a biased v variable region repertoire of the tcr expressed by that subject or by all subjects expressing a given class ii haplotype however this explanation does not hold for the findings reported here because the irss were recognized by subjects of different mhc class ii haplotype the molecular basis of the preferred recognition of the irs by human cd4 cells could be due to the characteristics of the interaction of peptide epitopes with hla class ii molecule and or the structural properties of the ag molecule which may influence processing and presentation of certain sequence regions many peptide sequences can bind different dr alleles nonetheless the ability of a given ag sequence to bind most or all dr molecules does not suffice for a peptide to be an irs for example see manfredi et al j immunol.