Once at the site of tissue injury, the neutrophil must first recognize any foreign substances before phagocytosis can occur. Like most phagocytic cells, neutrophils have a variety of receptors on their cell membrane that can recognize and bind to bacteria, foreign organisms, and other infectious agents (Fig 10.7). Some of these organisms and agents bind directly to neutrophils (no modifications of their surfaces are required), whereas others must be opsonized (coated with antibodies or complement) to make them more attractive to the neutrophil. The most common receptors used by neutrophils during phagocytosis include the following.  Fc receptors on the neutrophil surface bind to the ex posed Fc region of IgG antibodies that coat bacterial surfaces (see Fig 10.7). Binding of IgG-coated bacteria activates the neutropil’s phagocitic activity and causes a rapid surge in intracellular metabolism.
 Complement receptors (CRs) facilitate binding and uptake of immune complexes that are opsonized by active C3 complement protein, namely, C3b. Binding of bacteria or other C3b-coated antigens to CRs triggers phagocytosis, resulting in activation of a neutrophil’s lytic pathways and respiratory burst reactions.
 Scavenger receptors (SRs) are a structurally diverse group of transmembrane glycoproteins that bind to modified (acetylated or oxidized) forms of low-density lipoproteins (LDLs), polyanionic molecules that are often on the surface of both Gram-positive and Gram-negative bacteria and apoptotic bodies. Binding of these receptors increases the phagocitic activity of neutrophils.
 Toll-like receptors, also known as pattern recognition receptors (PRRs), are neutrophil receptors that recognize pathogenic molecules such as endotoxins, lipopolysaccharides, peptidoglycanes, and lipoteichoic acids that are arranged in predictable pathogen-associated molecular patterns (PAMPs) and are commonly expressed on bacterial surfaces and other infectious agents. Like other phagocytic cells, neutrophils possess a variety of toll-like receptors that recognize PAMPs. Binding of bacterial
FIGURE 10.7 • Neutrophil phagocytosis. a. Phagocytosis begins with recognition and attachment of foreign material (antigen), mainly by Fc receptors that interact with the Fc region of antibodies bound to the antigen. b. The antigen is then engulfed by pseudopods of the neutrophil. c. As the pseudopods come together and fuse, the antigen is internalized. d. Once the phagosome is formed, digestion is initiated by activation of membrane-bounded oxidases of the phagosome. e. Next, both specific and azurophilic granules fuse with the phagosome and release their contents, forming a phagolysosome. This fusion and release of granules is called degranulation. f. The enzymatic contents of the granules are responsible for killing and digesting the microorganism. The entire digestive process occurs within the phagolysosome, which protects the cell from self-injury. g. The digested material is either exocytosed into the extracellular space or stored as residual bodies within the neutrophil.
antigens to these receptor causes phagocytosis and the release of cytokines such as interleukin-1 (IL-1), interleukin-3 (IL-3), and tumor necrosis factor  (TNF-) from the neutrophil. IL-1, historically known as a pyrogen (fever-causing agent), induces synthesis of prostaglandins, which in turn act on the thermoregulatory center of the hypothalamus to produce fever. Therefore, fever is a consequence of acute reaction to invading pathogens that cause a massive neutrophilic response.