CD49d is a 150 kD glycoprotein, also known as integrin alpha4 or VLA-4 alpha chain. It is expressed as a heterodimer with either of two beta chains, beta1 or beta7. The alpha4beta1 integrin (also known as VLA-4, CD49d/CD29) is expressed on peripheral T and B lymphocytes, thymocytes, and monocytes, while the alpha4beta7 integrin (also known as LPAM-1) is expressed on peripheral lymphocytes. The alpha4beta1 and alpha4beta7 integrins mediate cell-cell and cell-matrix interactions by interacting with the ligands CD106 (VCAM-1), MAdCAM-1, and fibronectin. Soluble MRalpha4-1 antibody can partially inhibit in vitro binding of VCAM-1 and mast cells to fibronectin and inhibit mast cell degranulation. Immobilized MRalpha4-1 antibody can enhance mast cell degranulation in vitro.