Phagocytosis: A Fundamental Process in Immunity

Figure 4

Cooperation among phagocytic receptors. (a) Most phagocytic receptors, such as receptors for antibody (FcγRIIa) and receptors for complement (Integrin CR3) are small molecules that extend only few nanometers from the plasma membrane. In contrast, transmembrane glycoproteins, such as phosphatases CD45 (CD45RO and CD45RA isoforms), are much longer and usually rigid molecules. (b) In the resting state, receptors cannot diffuse freely throughout the membrane. Their movement is restricted by fences of transmembrane glycoprotein “pickets” attached to an actin mesh. (c) FcγR aggregation triggers an inside-out signal that activates integrins. FcγR-induced activation of phospholipase C (PLC) produces diacylglycerol (DAG) that leads to activation of CalDAG (a Rap GEF), which in turn activates Rap. Activated Rap (Rap GTP) is responsible for integrin activation by disrupting interactions between integrin subunits and promoting binding to talin, vinculin, and the actin cytoskeleton.

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