Our immune system has been shaped by constant challenge from the microbial world, a concept that has inspired our cover image this month. In keeping with this theme, on page 675, Michael Katze and colleagues discuss the 'arms race' between viruses and the host interferon response; they propose new approaches to therapeutic intervention that will help the host to gain the upper hand.

For some infections, such as HIV, it seems that the virus is a clear winner of the evolutionary race — in spite of the best efforts of innate and adaptive immunity. These infections present a tough challenge for vaccine design. At present, HIV-vaccine efforts are focused on CD8+ T cells, but have we been too quick to forget about antibodies? In an Opinion article on page 706, Dennis Burton reassesses the antiviral activities of antibodies in the context of a complex immune response that comprises many components. He argues that successful antiviral vaccines of the future will need antibodies, CD4+ T cells and CD8+ T cells to work together.

Unlike HIV, most acute viral infections are resolved by efficient antibody responses of high affinity and switched isotype. This requires the diversification of antibody genes by specialized mechanisms that are unique to B cells. Surprisingly, there is considerable variation in the mechanisms that generate antibody diversity between vertebrate groups. On page 688, Martin Flajnik discusses the unique immunobiology of chickens, sheep, rabbits and sharks, amongst others, and gives a new perspective on the emergence of adaptive immunity.

Of course, invertebrates have been doing just fine without T cells or B cells. A highlight article, on page 632, discusses recent insights into innate immunity in worms, which reveal pathways that are conserved in humans.