Ab-4 is important for identifying macrophages in tissue sections. It stains macrophages in a wide variety of human tissues, including Kupffer cells and macrophages in the red pulp of the spleen, in lamina propria of the gut, in lung alveoli, and in bone marrow. Ab- does not react with APCs, follicular dendritic cells, interdigitating cells, myeloid precursors and peripheral blood granulocytes. It also reacts with plasmacytoid T cells which are supposed to be of monocyte/macrophage origin. It shows strong granular cytoplasmic staining of chronic and acute myeloid leukemia and also reacts with rare cases of true histiocytic neoplasia. Tumors of lymphoid origin are usually not stained.