Page 158 - Veterinary Immunology, 10th Edition

VetBooks.ir Macrophages

Macrophages not only detect and kill invading microbes, but they
also produce a mixture of cytokines that promotes both innate and

adaptive immune responses; they can control inflammation; and
they contribute directly to the repair of damaged tissues by
removing dead, dying, and damaged cells and assist the healing
process. Their name is derived from the fact that they are “large-
eating” cells (Greek macro, phage).

Macrophages are a diverse family of cells that reside in different
tissues, and as a result differ greatly in their transcriptional profiles
and in their functions. They can change their activation state and

phenotype in response to signals, especially cytokines, from other
cells and from their local environment. This adaptability and
resulting phenotypic diversity has given rise to a confusing
nomenclature.
One population of bone marrow-derived macrophages circulates

in the bloodstream, where they are called monocytes. It was long
believed that all tissue macrophages were derived from these blood
monocytes. Recent studies, however, have shown that some tissue

macrophages such as those in the brain develop directly from
embryonic precursors. Their stem cells enter tissues in embryonic
life and persist in adults by self-renewal. Macrophages are found
throughout the body in connective tissue, where they are called
histiocytes; those found lining the sinusoids of the liver are called

Kupffer cells; those in the brain are microglia. The macrophages in
the alveoli of the lungs are called alveolar macrophages, whereas
those in the capillaries of the lung are called pulmonary

intravascular macrophages. Some macrophages can develop into
dendritic cells. Large numbers are found in the sinusoids of the
spleen, bone marrow, and lymph nodes. Irrespective of their name,
origin, or location, they are all considered macrophages and
collectively form the mononuclear phagocyte system (Fig. 6.1).

158