Macrophages can be activated to perform Functions.

http://en.wikipedia.org/wiki/Phagocyte

 Macrophages can be activated to perform functions that a resting monocyte cannot.T helper cells (also known as effector T cells or Th cells), a sub-group of lymphocytes, are responsible for the activation of macrophages. Th1 cells activate macrophages by signaling with IFN-gamma and displaying the protein CD40 ligand.Other signals include TNF-alpha and lipopolysaccharides from bacteria. Th1 cells can recruit other phagocytes to the site of the infection in several ways. They secrete cytokines that act on the bone marrow to stimulate the production of monocytes and neutrophils, and they secrete some of the cytokines that are responsible for the migration of monocytes and neutrophils out of the bloodstream.Th1 cells come from the differentiation of CD4+ T cells once they have responded to antigen in the secondary lymphoid tissues.Activated macrophages play a potent role in tumor destruction by producing TNF-alpha, IFN-gamma, nitric oxide, reactive oxygen compounds, cationic proteins, and hydrolytic enzymes.