Immunity in this framework is non-waning: exposure to a strain yields consistent protection from future infection over the lifespan of the individual. Moreover, this protection is trichotomous: an individual can either have no protection from a given strain (it has not seen any of the alleles before), complete protection (it has seen this exact combination of alleles before), or a set point in-between according to the parameter γ (it has seen at least one, but not all alleles before). Put another way, we do not distinguish between loci, assuming that sharing an allele at one locus is functionally identical to sharing an allele at any other locus, or indeed all other loci except one.