For any ring R, there is a covariant functor associated to every pp formula and pp pair over R. These functors have an algebraic characterisation. However, for algebras over a field, pp formulas and pp pairs also give rise to contravariant functors. These are the "anihilator functors". I will explain the relationship between the annihilator functor and the covariant functor given by a pp pair, and give the corresponding algebraic conditions which characterise the annihilator functors. Furthermore, I will explain how this impacts on the definability of classes of modules.