This volume presents a collection of reviews derived from work presented at the Aegean Conference: "4th Crossroads between innate and adaptive immunity". This meeting was the fourth in a series, and assembled a team of scientists working on mechanisms by which the innate immune system of the host senses pathogens, the cellular and signaling networks that orchestrate the innate response and antigen presentation and adaptive immunity. The importance of the crosstalk between innate immunity and the adaptive immune response has only recently started to be appreciated. Although it is well recognized that dendritic cells, NK cells, NK-T cells and T cells are all critical for the host response to pathogens, the respective fields that study the biology of these immune cells tend to exist in parallel worlds with minimum exchange of information and ideas. This fragmentation hinders the integration of these fields towards a unified theory of host response. The Aegean Conference "Crossroads between Innate and Adaptive Immunity" brought together leading international scientists and experts to address critical areas of Innate and Adaptive immunity something necessary for the development of more efficient scientific exchange and crosspollination between these fields. This conference attracted scientists from all over the world to discuss their latest findings on the various aspects of Innate and Adaptive immunity. The conference had limited participation and a scientific and social program that maximized scientific interchange through lecture presentations, poster sessions and informal discussions. ?