Mast cell tumours arise from mast cells, a type of immune cell found in connective tissues throughout the body that normally plays a role in allergic and inflammatory responses. These cells contain granules filled with histamine and other inflammatory chemicals, which can be released by the tumour, occasionally causing redness, swelling, or gastrointestinal upset. Mast cell tumours are the most common skin cancer in dogs and typically present as a lump in or under the skin. Low-grade tumours, as classified by the Kiupel grading system, generally behave in a relatively benign fashion — they grow slowly, are less likely to spread, and carry an excellent prognosis when completely removed surgically. Certain breeds, including Boxers, Pugs, Labrador Retrievers, and French Bulldogs, are predisposed. Even with a low-grade classification, complete surgical removal with adequate margins is important to prevent local recurrence.