Immune cells in mammary tumours arising from PyMT expression. (a-e) Immunostaining for various bone marrow derived cells, and (f) toludine blue staining for mast cells in MMTV-PyMT mammary tumors. Macrophages are commonly present within and around tumors (a, b). Arrows indicate macrophages (b), T-cells (c), B-cells (d), neutrophils (e), and mast cells (f). T- and B-cells are often present as groups of mixed lymphocytes at the borders of lesions, and are more frequent than neutrophils unless necrosis is present. Mast cells are the most rare, and are usually solitary or in groups of two to three cells.