CD4 and chemokine co-receptors for HIV may be expressed on cells at multiple layers of this hierarchy. The structure and function of these organs may be compromised by a variety of direct and/or indirect effects of HIV infection43, 44. In the young adult, bone marrow-derived T progenitor cells continue to mature, albeit at a reduced rate, through the thymus and the peripheral lymphoid system is continuously seeded with naive T cells bearing a maximally diverse TCR repertoire. When HIV is introduced into the body (stage a), it is probably concentrated within draining lymph nodes and presented as an antigen, resulting in enhanced movement of T cells into nodes and vigorous T-cell proliferation. Immune activation caused by HIV will be associated, by direct and indirect means, with accelerated destruction of T cells (stage b). To compensate for peripheral lymphoid depletion, 'sensor cells'68 in lymphoid organs may produce positive regulatory signals (for example, IL-7) which promote enhanced T-cell production at multiple levels of the haematopoietic tree (stage c). These signals may in turn accelerate virus replication and the emergence of highly cytopathic variants, leading to the destruction of key progenitor cells in the bone marrow, the thymus and/or the peripheral lymphoid system (stage d). Impaired production of new cells from these organs would then result in collapse of the immune system and conversion of a high turnover state to one of low turnover.