The extent to which the functional heterogeneity of Mvarphis is dependent on the differentiation of functional sublineages remains unresolved. One alternative hypothesis proposes that Mvarphis are functionally plastic cells, which are capable of altering their functional activities progressively in response to progressively changing signaling molecules generated in their microenvironment. This "functional plasticity" hypothesis predicts that the functionally polarized Mvarphis in chronic pathologies do not represent Mvarphi sublineages but rather, are mutable phenotypes sustained by chronic signaling from the pathological environment. Solid TAMvarphis are chronically polarized to provide activities that support tumor growth and metastasis and suppress adaptive immune responses. In support of the functional plasticity hypothesis, administration of slow-release microsphere-encapsulated IL-12 successfully reprogrammed TAMvarphis in situ, reducing Mvarphi support of tumor growth and metastasis and enhancing Mvarphi proimmunogenic activities. Increased knowledge of how Mvarphi function is regulated and how polarized Mvarphis can be reprogrammed in situ will increase our ability to control Mvarphi function in a variety of pathological states, including cancer and chronic inflammatory disease.