This introductory review serves to introduce the reader to several families of cellular chemoattractants including cytokines known as chemokines, as neuropeptides, components of the complement cascade, and cyclooxygenase pathways. These ligands share the capacity to utilize seven transmembrane (STM) G protein-coupled receptors. As is true of other cyto-
kines, these ligands are multifunctional and, in addition to mobilizing inflammatory cells to sites of infection and injury, act on a wide spectrum of progenitor cells and on nonleukocytes. Consequently, some of these ligands participate in fetal development, others on hematopoiesis on trafficking and homing of cells to tissues and organs, on the induction of inflammatory and immune responses, and on regulating cell adhesion and angiogenesis. We will also review reports that numerous viruses and microorganisms have adapted and learned to circumvent these mediators of host defense by producing agonists or antagonists or, as illustrated by HIV-1, by using chemokine receptors as entry sites for cellular invasion.