The effect of HLA-C mismatching on CTLp frequency. CTLp frequencies of 30 pairs with patient HLA-C locus incompatibilities are shown above. Mean CTLp frequencies differed significantly between those with only HLA-C mismatches (1:316,500) and those with further HLA class I mismatches (1:47,110). This indicates that HLA-C alloreactive T cells are found at a lower frequency than those at HLA-A and -B. The limit of sensitivity of this assay is 1 patient-specific donor CTLp/5 × 10⁵ or higher. Mean CTLp frequencies differed significantly between groups according to the Mann-Whitney test.