The Bayeux Tapestry depicts the Norman invasion of England. This month, we have based our cover on this iconic image to depict another type of invasion — that of the gut by intestinal pathogens — and the battle given by intestinal intraepithelial lymphocytes against pathogenic invaders (inspired by the Review article on page 445).

The epic story of immune defence starts with the development of capable soldiers. On page 478, Howard Cedar and Yehudit Bergman describe the complex epigenetic mechanisms that determine the fate of haematopoietic stem cells and regulate their ranking in the innate and adaptive immune defence hierarchy.

The training of T cell soldiers gets more sophisticated. It is the thymus that educates and selects the most efficient T cells to be sent to the periphery. On page 469, Paul Love and Avinash Bhandoola describe the signalling events that direct T cell progenitors to the thymus, regulate their intrathymic wandering (during which they are positively or negatively selected) and, finally, permit the exit of self-tolerant but antigen-reactive T cells into the blood.

The involvement of the thymus in T cell development is now well established, but was unknown in the 1950s. A Timeline article on page 489 by Jacques Miller, who discovered the immunological function of the thymus 50 years ago, summarizes the milestones of thymic research.

Once in the periphery, T cells must be primed to start battling against pathogens. Dendritic cells trap antigens to be presented to T cells and produce cytokines to shape T cell function in response to the environment. A Poster by Miriam Merad that depicts the diverse functions of dendritic cells in the control of adaptive immunity is freely available at http://www.nature.com/nri/posters/dendriticcells, thanks to support from BioLegend.