Page 169 - Veterinary Immunology, 10th Edition

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FIG. 6.8 The progressive activation of macrophages can involve
three pathways. Innate activation occurs in response to TLR
ligation. Additionally, macrophages can become classically
activated M1 cells by exposure to microbial products and
subsequent exposure to Th1 cytokines such as IFN-γ. Alternatively,
they may undergo “alternative activation” on exposure to Th2
cytokines and become M2 cells.

Box 6.1

Circadian Rhythm and Monocytes

There is a daily oscillation of the numbers of monocytes present in
mouse tissues. The numbers tend to increase while an animal is
resting and decrease when the animal is active. Monocytes also

express an intrinsic circadian clock where their gene expression
pattern varies over 24 hours. The monocytes that exhibit the
diurnal rhythm in tissues are “inflammatory” monocytes. At the

beginning of the animal's resting time (daytime for a mouse), the

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