Being in the right place at the right time is important for the development of effective immune responses, as discussed in two articles this month.

For developing thymocytes, interactions with thymic stromal cells as they migrate though the thymus are essential for the development and selection of mature T cells. However, the signals that control the migration of thymocytes into and within the postnatal thymus have remained surprisingly elusive, as discussed by Howard Petrie on page 859.

On page 867, Ulrich H. von Andrian and Thorsten R. Mempel describe how the specialized architecture and coordinated movement of cells within lymph nodes enable them to carry out their central roles in the immune system. The amazing images of the microcirculation of the lymph node that can be obtained using fluorescent dyes have inspired our cover this month.

It is not only the lymph nodes that have unique characteristics specific for their function. As discussed on page 879 by J. Wayne Streilein, the anatomical, cellular and molecular features of the eye combine to protect this organ from the otherwise potentially destructive effects of inflammation. An increased understanding of the basis of the immune-privileged status of the eye could lead to the development of new drugs for immunopathogenic disorders and that promote the survival of transplants.

In another article with an applied focus, on page 851, Mario Stevenson describes how RNA interference — a process that targets specific RNA sequences for degradation — could be used not only to learn more about the biology of HIV-1, but also potentially to develop new therapies for treating HIV-1 infection.