To produce a comprehensive overview of macrophages and related cell types in a short review volume is an impossible task. When I selected the topics to be included, some equally important areas were omitted by necessity, and for this I apologize. My choices have been somewhat eclectic, touching subjects of personal interest (such as osteoclast biology and macrophage electrophysiology) or of current fashion (apopto sis, antigen processing, cell adhesion molecules). The book has also had to encompass areas of a more general flavor to provide balance for the general reader (such as reviews of macrophage development, heterogeneity, and function, and of the surface molecules expressed by macrophages). I thank all the authors for their prompt sub missions; all have been of high quality, and my editorial tasks, thankfully, have been minimal. Michael A. Horton London, United Kingdom ix Contents Chapter J An Overview of Receptors of MPS Cells lain Fraser and Siam on Gordon 1. Introduction. 1 2. The Mononuclear Phagocyte System. 2 3. Diversity of Macrophage Plasma Membrane Receptors. 6 3. 1 A Structural Approach to Classification. 6. 8 3. 2 Multisubunit Receptors 3. 3 Soluble Receptors. 9 3. 4 Lectins and Lectin-Like Receptors. 12 4. Functions and Selected Examples. 14 4. 1 Growth, Differentiation, and Modulation. 14 4. 2 Cell-Cell and Cell-Matrix Interactions. 16 4. 3 Endocytosis and Scavenger Receptors. 16 4. 4 Secretory Responses and Biosynthesis of Effector Molecules. 17 5. Concluding Remarks. 17 6. References. 18.