Recognizes the L1 or Calprotectin molecule, an intra-cytoplasmic antigen comprising of a 12kDa alpha chain and a 14kDa beta chain expressed by granulocytes, monocytes and by tissue macrophages. Macrophages usually arise from hematopoietic stem cells in the bone marrow. Under migration into tissues, the monocytes undergo further differentiation to become multifunctional tissue macrophages. They are classified into normal and inflammatory macrophages. Normal macrophages include macrophages in connective tissue (histiocytes), liver (Kupffer’s cells), lung (alveolar macrophages), lymph nodes (free and fixed macrophages), spleen (free and fixed macrophages), bone marrow (fixed macrophages), serous fluids (pleural and peritoneal macrophages), skin (histiocytes, Langerhans’s cell) and in other tissues. Inflammatory macrophages are present in various exudates. Macrophages are part of the innate immune system, recognizing, engulfing and destroying many potential pathogens including bacteria, pathogenic protozoa, fungi and helminthes. This MAb reacts with neutrophils, monocytes, macrophages, and squamous mucosal epithelia and has been shown as an important marker for identifying macrophages in tissue sections.