**1. Introduction**

Mast cells are originated from hematopoietic stem cells in bone marrow as similar to other granulocytes. However, mast cells are very unique because they are released from bone mar‐ row as undifferentiated progenitor cells to the circulation, and their final maturation is com‐ pleted in the peripheral tissues. We find mast cells in most of the tissues and organs in our body, particularly in interstitial connective tissues of each organ being close to blood vessels. Mast cells play crucial roles in innate immunity against parasites and microbes that is essen‐ tial for host defense in humans and animals. In acquired immunity, activation of mast cells is induced by cross‐linkage of IgE that binds to high‐affinity IgE receptors after antigen expo‐ sure. Moreover, some chemicals and toxins as well as physical (i.e., scratching and heat) and neurogenic stimuli trigger activation of mast cells. Various chemical mediators and cytokines are released from mast cells after their degranulation, and sometimes initiate allergic inflam‐ mation and itch sensation. In canine medicine, serious involvement of mast cells in allergic

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Typical histologic features of surgically removed canine MCTs stained with toluidine blue solution. A mass is consisted with numbers of differenced mast cells that show metachromasia. Connective tissues colored in light blue can be identified.

diseases has been identified, and mast cells are estimated as the most important target in medical treatment of allergy. Although malignancies of mast cell are uncommon disorders in humans, veterinary clinicians frequently encounter mast cell tumors (MCTs) in dogs and cats. The frequency of cutaneous MCTs has been reported to reach 20% of all tumors raised in the skin of dogs. Most of the malignant mast cells existed in a mass have granules in their cytosol, containing pruritogens, inflammatory factors, and various proteases (**Figure 1**). In this chapter, recent information on both basics in mast cell biology and clinical approaches for canine MCTs is outlined.
