Various receptor systems have been described on vascular cells that participate in the recognition, activation, and clearance of components involved in humoral defense. Among these molecules are receptors for the complement protein, C1q. Four C1q binding proteins or receptors have been described to date: cC1q-R/CR or collectin receptor, a 60-kDa calreticulin (CR) homolog, which binds to the collagen-like tail of C1q; gC1q-R/p33, a 33kDa homotrimeric protein with high affinity for the globular heads of C1q; C1q-Rp (CD93), a 129 kDa O-sialoglycopro-tein involved in phagocytosis; and CR1 (CD35), the receptor for C3b. In addition to C1q, the collectin receptor (cC1q-R/CR) is also able to bind members of the collectin family of proteins including surfactant proteins A and D (SP-A and SP-D) and mannan binding lectin (MBL). Similarly, the gC1q-R/p33 molecule can bind plasma proteins other than C1q, including high-molecular-weight kininogen (HK) and FXII, which may play a significant role in the pathogenesis of thrombosis and atherosclerosis. In addition, gC1q-R/p33 also binds a number of bacterial and viral proteins including protein A of Staphylococcus aureus, known to be associated with bacterial endocarditis. Whereas CD93 and CD35 are bona fide transmembrane proteins and can signal directly, cC1q-R/CR and gC1q-R/p33 each lack a transmembrane segment in their respective sequences and therefore rely on a docking/signaling partnership with other transmembrane proteins in order to relay their message across the membrane (e.g., CD91 and cC1q-R/CR; b1-integrin and gC1q-R/p33).
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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.