CD163 has been identified as an acute phase-regulated transmembrane protein whose function is to mediate the endocytosis of haptoglobin-hemoglobin complexes. This receptor is expressed on the surface of monocytes with low expression and on tissue macrophages, histiocytes with high expression. Staining with anti-CD163 has been helpful to distinguish synovial macrophages from synovial intimal fibroblasts in rheumatoid arthritis, where specificity for macrophages was found to be superior to that of CD68.