Plays a role in virion attachment to host B-lymphocytes, through binding to leukocyte antigen (HLA) class II and subsequently participates in fusion of the virion with host membranes. May act as a tropism switch that directs fusion with B-lymphocytes and inhibits fusion with epithelial cells (By similarity). Forms a complex with gp25 and gp85 via its N-terminus; this complex is used for invasion of B-lymphocytes. virions synthesized in B-lymphocytes contain a lower amount of gp42 due to sequestration by cellular HLA class II protein, whereas virions made from epithelial cells has a higher amount of gp42. Membrane; Single-pass membrane protein (By similarity). The C-lectin type domain is essential for virion-induced membrane fusion. Belongs to the epstein barr virus gp42 family.