• Mast cells are widely distributed in connective tissues and participate in both acute and chronic inflammatory reactions. Mast cells arise from precursors in the bone marrow. They have many similarities with circulating basophils, but they do not arise from basophils, are tissue-resident, and therefore play more significant roles in inflammatory reactions in tissues than basophils do. Mast cells (and basophils) express on their surface the receptor FcεRI, which binds the Fc portion of IgE antibody. In immediate hypersensitivity reactions, IgE bound to the mast cells’ Fc receptors specifically recognizes antigen, and in response the cells degranulate and release mediators, such as histamine and prostaglandins (Chapter 5). This type of response occurs during allergic reactions to foods, insect venom, or drugs, sometimes with catastrophic results (e.g., anaphylactic shock). Mast cells also are present in chronic inflammatory reactions, and because they secrete a
Fig. 3.21 A focus of inflammation containing numerous eosinophils.
plethora of cytokines, they can promote inflammatory reactions.
• Although neutrophils are characteristic of acute inflammation, many forms of chronic inflammation, lasting for months, continue to show large numbers of neutrophils, induced either by persistent microbes or by cytokines and other mediators produced by activated macrophages and T lymphocytes. In chronic bacterial infection of bone (osteomyelitis), a neutrophilic exudate can persist for many months. Neutrophils also are important in the chronic damage induced in lungs by smoking and other irritant stimuli (Chapter 13). This pattern of inflammation has been called acute on chronic.
Granulomatous inflammation is a form of chronic inflammation characterized by collections of activated macrophages, often with T lymphocytes, and sometimes associated with central necrosis. The activated macrophages may develop abundant cytoplasm and begin to resemble epithelial cells, and are called epithelioid cells. Some activated macrophages may fuse, forming multinucleate giant cells. Granuloma formation is a cellular attempt to contain an offending agent that is difficult to eradicate. In this attempt there is often strong activation of T lymphocytes leading to macrophage activation, which can cause injury to normal tissues.
There are two types of granulomas, which differ in their pathogenesis.
Immune granulomas are caused by a variety of agents that are capable of inducing a persistent T cell–mediated immune response. This type of immune response produces granulomas usually when the inciting agent cannot be readily eliminated, such as a persistent microbe or a self antigen. In such responses, macrophages activate T cells to produce cytokines, such as IL-2, which activates other T cells, perpetuating the response, and IFN-γ, which activates the macrophages.
Foreign body granulomas are seen in response to relatively inert foreign bodies, in the absence of T cell– mediated immune responses. Typically, foreign body granulomas form around materials such as talc (associated with intravenous drug abuse), sutures, or other fibers that are large enough to preclude phagocytosis by a macrophage but are not immunogenic. Epithelioid cells and giant cells are apposed to the surface of the foreign body. The foreign material can usually be identified in the center of the granuloma, particularly if viewed with polarized light, in which it may appear refractile.
MORPHOLOGY In the usual H&E stained tissue samples (