Plays a role in the dissolution of the outermost membrane of extracellular enveloped virions (EEV) to allow virion entry into host cells. Participates also in wrapping intracellular mature virions (IMV) to form intracellular enveloped virions (IEV) (By similarity). Interacts with A33; this interaction is required for efficient targeting of A33 and B5 into enveloped virions. Interacts with A34; this interaction is required for the correct glycosylation, trafficking and stability of A34 and B5 incorporation into extracellular enveloped virions. Interacts with envelope phospholipase F13. B5 is found on enveloped virion (EV) membranes. Belongs to the receptors of complement activation (RCA) family.