Which Cluster Designation must Epstein-Barr Virus (EBV) attach to in order to infect cells?

Epstein-Barr Virus (EBV) specifically uses CD21, also known as complement receptor 2 (CR2), as its primary receptor for attachment and entry into host B cells. CD21 is found on the surface of B lymphocytes and plays a crucial role in the immune response, particularly in the activation and differentiation of B cells. Once EBV binds to CD21, it can initiate the process of viral entry, leading to infection. This receptor interaction is a key step in the pathogenesis of EBV-related diseases, including infectious mononucleosis and certain lymphoproliferative disorders.

The other options listed are not involved in EBV's mechanism of entry. CD4 is primarily a receptor for the Human Immunodeficiency Virus (HIV) and is important for T cell activation. CD19 is a component of the B cell receptor complex but doesn't serve as a receptor for EBV. CD45 is a receptor-type protein-tyrosine phosphatase found on all leukocytes, and while important in the activation and signaling of cells, it does not facilitate EBV entry. Thus, the use of CD21 by EBV is the critical factor for the virus to effectively infect B cells.