Low density lipoprotein receptor-related protein 1 (LRP1), also known as alpha-2-macroglobulin receptor (A2MR), apolipoprotein E receptor (APOER) or cluster of differentiation 91 (CD91), is an endocytic receptor involved in several cellular processes. LRP1 / CD91 is a member of the cluster of differentiation (CD) system. The cluster of differentiation (CD) system is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 32 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. LRP1 / CD91 is involved in endocytosis and in phagocytosis of apoptotic cells and is required for early embryonic development. LRP1 / CD91 is also Involved in cellular lipid homeostasis, the plasma clearance of chylomicron remnants, activated LRPAP1 (alpha 2-macroglobulin), and the local metabolism of complexes between plasminogen activators and their endogenous inhibitors. LRP1 / CD91 May modulate cellular events, such as APP metabolism kinase-dependent intracellular signaling, neuronal calcium signaling as well as neurotransmission need the involvement of LRP1 / CD91.