Role of Macrophages in Host Defenses. Macrophages are generated from bone marrow precursor cells in the presence of stem cell hematopoietic factors (such as granulocyte-macrophage colony stimulating factor/GM-CSF and macrophage colony stimulating factor/M-CSF). The activation of macrophages (in the presence of specific receptors and T lymphocytes as shown in the figure- CD40/CD40L, CD28/B7, interleukin-interleukin receptor etc) results in functional consequences such as microbicidal activity directed especially towards intracellular pathogens and aided by the expression of peroxides and superoxide radicals. These processes are defective in CGD (see text). Other secretory effects of macrophages result in the production of a plethora of mediators that assist in immunity (these are shown in the figure and include complement, enzymes, interleukins, interferons and growth factors).