Mononuclear phagocytes and inflammation

During an acute or chronic inflammation, the number of macrophages may increase and both the degree of the increase and the time at which it occurs are dependent on the nature of the stimulus and the type of tissue in which the inflammation occurs. Circulating monocytes are mainly responsible for the

Table 1 Ontogeny of mononuclear phagocytes

Site of formation of mononuclear phagocytes

Yolk sac Fetal liver

Bone marrow

Localization of mononuclear phagocytes at other sites

Circulating monocytes Osteoclasts

Time of gestation

Mouse

Days 7-12

Day 11 to 2nd week postnatal After days 11-12 After day 15 At birth

Human

Days 21-63 5th week to term After weeks 7-8 After weeks 8-9 After week 11

increase in the number of macrophages in the inflammatory exudate, and transitional forms between monocytes and resident macrophages are present in such exudates and are called exudate macrophages. There is also an increase of the number of dividing mononuclear phagocytes in an inflammatory exudate and in granulomas, due to the recruitment of immature mononuclear phagocytes from the bone marrow.

Alterations in the functional state of the (exudate) macrophages include their locomotion, chemotactic response, endocytic, microbicidal, cytostatic or cytotoxic activity, as well as their secretory state, their adherence to and spreading on a surface, and their cooperation with T and B lymphocytes. The inflammatory stimulus may act directly upon the macrophages and change their functional state, and such a change can also be mediated by cytokines secreted by lymphocytes or macrophages (i.e. an autocrine regulation), or by serum proteins such as immunoglobulins or complement.

Two other types of cell often present in chronic inflammatory lesions are the epithelioid cell and the multinucleated giant cell, both of which also derive from monocytes. Epithelioid cells in granulomas have characteristics that resemble those of macrophages. Multinucleated giant cells are formed by fusion of (exudate) macrophages, and not by nuclear division without cytoplasmic division. There are two types of multinucleated giant cell: the Langerhans type with relatively few nuclei arranged peripherally in the cytoplasm, and the foreign-body type with many nuclei dispersed throughout the cytoplasm.

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