Current issues in antigen presentation--focus on the dendritic cell

A personal view of some of the major current issues in antigen presentation, which focuses on the biology of the dendritic cell. The question of dendritic cell heterogeneity is discussed and the inclusion of the "plasmacytoid" dendritic cell in this family is questioned. Defining the molecules involved in dendrite formation may ultimately provide the only reliable signature of the dendritic cell. The role of the dendritic cell as a key link between innate and adaptive immunity is here revisited, and the direct and indirect interactions between dendritic cells and microorganisms are discussed. Reactive oxygen species may be key modulators of dendritic cell function, although it remains unclear whether direct or indirect mechanisms are more important. Study of the cell biology of antigen processing reveals a tightly regulated complex system involving multiple proteinases, adapted to the most efficient presentation of invading microorganisms and playing a key role in shaping the T cell repertoire.