Immunological aspects of chronic inflammation and repair.

Macrophages and lymphocytes play essential roles in cell-mediated mechanisms involved in chronic inflammation. During the development of specific immunity, macrophages are required to process and present antigen to immunocompetent T cells. They also serve as "accessory" cells to lymphocytes by releasing soluble factors involved in host defense. In order to carry out their role more effectively, macrophages have the property of being activated, a process that provides them with greater ability to phagocytose and kill ingested microorganisms. Activated T cells serve as key effector cells that are capable of producing lymphokines which modify the behavior of other cells. In this way, they are able to facilitate or suppress an immune response. Chronic inflammation is often associated with irreversible destruction of parenchymal tissue, and fibrous connective tissue fills the resultant defect. Proliferation of fibroblasts, collagen production, and neovascularization are enhanced by the secretion of cytokines by T cells and macrophages.