Mast cells are present in most tissues and can form an important part of your cat's immune system. In the case of a mast cell tumor, the healthy cell begins to. Visceral form: As you might guess, mast cell tumors located internally are more serious than those located in the skin. The most common organs involved are the spleen, liver and intestine. Vomiting, appetite loss and weight loss are the most common symptoms associated with these g: facial | Must include: facial.