A mastocytoma or mast cell tumor is  a type of round-cell tumor consisting of mast cells. It is found in humans and many animal species; it also can refer to an accumulation or nodule of mast cells that resembles a tumor. Mast cells originate from the bone marrow and are normally found throughout the connective tissue of the body as normal components of the immune system. As they release histamine, they are associated with allergic reactions. Mast cells also respond to tissue trauma. Mast cell granules contain histamine, heparin, platelet-activating factor, and other substances. Disseminated mastocytosis is rarely seen in young dogs and cats, while mast cell tumors are usually skin tumors in older dogs and cats. Although not always malignant, they do have the potential to be. Up to 25 percent of skin tumors in dogs are mast cell tumors, with a similar number in cats. Signs and symptoms
Humans
When mastocytomas affect humans, they are typically found in skin. They usually occur as a single lesion on the trunk or wrist. Although it is rare, mastocytomas are sometimes found in the lung. It can also affect children. Other animals
Mast cell tumors are known among veterinary oncologists as the great pretenders because their appearance can be varied, from a wart-like nodule to a soft subcutaneous lump (similar on palpation to a benign lipoma) to an ulcerated skin mass. Most mast cell tumors are small, raised lumps on the skin. They may be hairless, ulcerated, or itchy.
Mineralization of the tumor is common. In pigs and cattle, mast cell tumors are rare. They tend to be solitary and benign in pigs and multiple and malignant in cattle. Mast cell tumors are found in the skin of cattle most commonly, but these may be metastases from tumors of the viscera. Other sites in cattle include the spleen, muscle, gastrointestinal tract, omentum, and uterus. Dogs
Mast cell tumors mainly occur in older adult dogs, but have been known to occur on rare occasions in puppies. The following breeds are commonly affected by mast cell tumors:

Boxer
Staffordshire bull terrier
Bulldog
Basset hound
Weimaraner
Boston terrier
Great Dane
Golden retriever
Labrador retriever
Beagle
German shorthaired pointer
Scottish terrier
Pug
Shar pei
Rhodesian ridgeback

Cats
Two types of mast cell tumors have been identified in cats, a mast cell type similar to dogs and a histiocytic type that appears as subcutaneous nodules and may resolve spontaneously. Young Siamese cats are at an increased risk for the histiocytic type, although the mast cell type is the most common in all cats and is considered to be benign when confined to the skin.Mast cell tumors of the skin are usually located on the head or trunk. Gastrointestinal and splenic involvement is more common in cats than in dogs; 50 percent of cases in dogs primarily involved the spleen or intestines. Gastrointestinal mast cell tumors are most commonly found in the muscularis layer of the small intestine, but can also be found in the large intestine.