CD4 and infection: Although CD134 is expressed at lower levels on macrophages than on activated CD4+ helper T cells, since macrophages are rich in CXCR4, a scenario may be envisaged whereby following initial infection and spread, the virus disseminates into compartments where CD134 expression is a limiting factor, principally cells of the monocyte/macrophage lineage, and as yet uncharacterised subpopulations of B220+ B cells and activated CD8+ T cells.