  In this work we introduce reciprocity functors, construct the associated
K-group of a family of reciprocity functors, which itself is a reciprocity
functor, and compute it in several different cases. It may be seen as a first
attempt to get close to the notion of reciprocity sheaves imagined by B. Kahn.
Commutative algebraic groups, homotopy invariant Nisnevich sheaves with
transfers, cycle modules or K\"ahler differentials are examples of reciprocity
functors. As commutative algebraic groups do, reciprocity functors are equipped
with symbols and satisfy a reciprocity law for curves.
