Historically, the study of immunology has focused to a large extent on the central role of antigen recognition and antibody structure and function in the response of the host to infection. In the last two decades, however, scientists have been turning their attention to the involvement of various cell types in host defense. Cells such as the macrophage, the natural killer cell, the neutrophil and other humoral factors make up this innate or "natural" immune system. They can take part in various immune responses without prior sensitization, and have important modulatory effects on later, specific responses. This volume provides a collection of reviews written by leading scientists. A broad range of topics is included relating the different soluble mediators collectively known as humoral factors. These include cytokines, tumor necrosis factors, the plasma complement system, and substances involved in coagulation, inflammation, and platelet adhesion. Humoral factors can destroy foreign material and damaged tissue and can act as mediators for and recruiters of other cells such as phagocytes and lymphocytes. In addition to playing a key role in host defense, humoral factors are important in homeostasis and in the long-term regulation of the immune system. Advanced students and researchers in immunology, cell biology, and medicine will find Humoral Factors to be a valuable source of current research into this exciting field.