Mast cell-mediated reactions of host defense and tissue injury: the regulatory role of eosinophil polymorphonuclear leukocytes.

Immunological stimulation of mast cells, by way of either IgE- or IgG-directed reactions, initiates the rapid release of an array of chemical mediators. The predominant local tissue effects of these mediators collectively constitute a defensive response of the host. The early humoral phase of defense is exemplified by the alterations in microvascular permeability induced by histamine which provide a local concentration of immunoglobulins and complement components. The later cellular phase of defense is composed of the PMN leukocytes that accumulate in response to mast cell-derived chemotactic principles and which phagocytose and degrade opsonized foreign material, thus eliminating the inciting stimulus. Of the several endogenous regulatory mechanisms which act to contain the immediate hypersensitivity reaction, the eosinophil has a special role since it is specifically attracted to sites of mast cell activation and has selective concentrations of several enzymes which degrade the mast cell-derived chemical mediators. Failure of the local regulatory processes can permit the mast cell responses of host defense to become pathological reactions leading to tissue injury by virtue of persistence of high levels of humoral mediators and/or increasing infiltration with PMN leukocytes.