The recruitment and activation of antigen-presenting cells are critical early steps in mounting an immune response. Many microbial components and endogenous mediators participate in this process. Recent studies have identified a group of structurally diverse multifunctional host proteins that are rapidly released following pathogen challenge and/or cell death and, most importantly, are able to both recruit and activate antigen-presenting cells. These potent immunostimulants, including defensins, cathelicidin, eosinophil-derived neurotoxin, and high-mobility group box protein 1, serve as early warning signals to activate innate and adaptive immune systems. We propose to highlight these proteins' unique activities by grouping them under the novel term 'alarmins', in recognition of their role in mobilizing the immune system.