Leukocyte extravasation from the blood into tissues plays a key role in innate and adaptive immunity and in inflammation. In order to understand the task of extravasation of a leukocyte traveling in the blood stream, Paul Kubes  recently suggested that we picture ourselves plunged into the water of a "roaring river," in order to imagine the high shear forces white cells are exposed to in the mainstream of the blood. In either scenario stopping and exiting the flow becomes an incredibly daunting task. Whereas we might struggle by various means using our hands and feet to exit the river, in the case of the white cell, a well-coordinated and extremely efficient sequence of leukocyte endothelial interactions has evolved allowing the leukocyte to marginate from the main bloodstream, to slow down and to "hold on" (adhere) to the endothelial vascular wall, and eventually to extravasate. Here, we will summarize the sequence of molecular steps known to govern leukocyte extravasation across the vascular wall to date.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.