Figure 1.

Alarmins link neutrophils (N) and dendritic cells (DC). Neutrophils infiltrating the site of tissue injury and infection release alarmins, such as defensins, cathelicidin, lactoferrin and HMGB1. Other cell types, such as injured or otherwise activated epithelial cells (Ep), can also contribute to the local production of alarmins. Locally generated alarmins directly recruit various subsets of leukocytes (including dendritic cells) based on their chemotactic effect. However, alarmins also activate various leukocytes, such as monocytes/macrophages (Mϕ)that are either resident or freshly recruited. Recruitment and activation of phagocytic cells by alarmins certainly contributes to the enhancement of the innate defense response through increasing the local number of phagocytes, augmenting phagocytosis, and killing of pathogens, and increasing the production of cytokines. Recruitment and activation of DCs by alarmins results in not only an increase in the local number of DCs but also in the phenotypic and functional maturation of DCs i.e. immature DC (iDC) to mature DC (mDC) conversion, which contributes to the enhanced uptake, processing and presentation (upon arrival at the secondary lymphoid organs) of antigen(s) to naïve T cells, and consequently the enhancement of adaptive immune responses.