Brigham and Women s Hospital, Harvard Medical School, Boston, Massachusetts
Vascular organisms have evolved elaborate systems to rapidly identify areas of injury and to begin the reparative process. Such systems signal through the blood to recruit selective elements to assist in this process, making the vasculature a key element in any reparative system. Collectively, these systems produce the inflammatory response, whereby areas of injury develop increased blood flow and leaky capillaries, augmenting an influx of cellular elements into the interstitial space to remove and repair damaged elements. Within the inflammatory response is the system of innate immunity. Areas of injury are also areas where barriers to microbes are breached. The system of innate immunity, a system to which complement belongs, recognizes and begins to check infection, at the same time attracting more host defenses to the cause. The distinction between inflammation and innate immunity is, in the end, artificial as the two processes involve a near-identical sequence of simultaneous steps and systems. Complement, as an example, plays major roles in the response to both infection and injury.
Despite the plethora of component proteins and protein fragments, there are only two functional parts to the complement system. One part includes the serum proteins and produces a cascade-like sequential activation of proteins in response to specific stimuli, "activation" (Figure 1). The other portion consists of cell surface proteins on immune competent cells and allows them to derive information from specific products of complement activation, "effector" (Figure 1). The link between the two parts is the pivotal step of the complement system: the production of a covalent bond between the C3b fragment of C3 and the target of complement activation, "opsonization" (Figure 1).
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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.