Interactions between acid-base groups of polyampholytes cause that protonation reactions occur in a pH range which may be very different from the pK values of isolated groups. If one introduces interactions between pairs (and possibly triplets) of individual acid-base groups, such titration behavior can be quantitatively understood in terms of Ising models which are familiar from statistical mechanics of magnets. For a finite number of sites, the Ising model reduces to the classical description of polyprotic acids and bases and yields explicit expressions for their pK values. For an infinite number of sites, the number of pK values becomes infinite, but the titration curves can be calculated in a straightforward fashion. In both situations, the titration curve of such a (possibly homogeneous) polyampholyte is identical to a titration curve of a mixture of monoprotic acids, provided the zeroes of the grand partition function of the corresponding Ising model are all negative and real. Under this very general condition, one has just observed that the popular affinity distribution description, which is commonly derived from a heterogeneous mixture of monoprotic acids and bases, also applies to homogeneous polyampholytes (or weak polyelectrolytes), From the present approach one can derive explicit expressions for affinity distributions and can show that a weak polyelectrolyte with an infinite number of identical, but interacting, acid-base groups has an affinity distribution which is continuous in a certain interval of pK values.