Page 143 - Veterinary Immunology, 10th Edition

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FIG. 5.10 The opsonization of a bacterium by antibodies and
complement. Binding of these ligands to their receptors triggers
ingestion and the respiratory burst. The antibody receptor is called
CD32, and the complement receptor is called CD35. Type 1
phagocytosis is mediated by antibodies through CD32. Type 2
phagocytosis is mediated by complement through CD35.

Extracellular Traps

Neutrophils trap and kill extracellular bacteria. Some neutrophils
undergo a form of cell death termed NETosis as an alternative to

apoptosis or necrosis. After activation by CXCL8 or
lipopolysaccharides, neutrophil oxidants release of the contents of
the azurophil granules. These enzymes cause chromatin
condensation and release strands of nuclear DNA and its associated
proteins into the extracellular fluid. These strands form a network

of extracellular fibers called neutrophil extracellular traps (NETs)
(Fig. 5.11). These NETs are coated with antimicrobial proteins,
including histones and granule components such as elastase,

myeloperoxidase, lactoferrin, and gelatinase. As a result, NETs not
only physically capture bacteria but also kill them. NETs are
abundant in sites of acute inflammation including mastitic milk. It
is believed that neutrophils can sense the size of their microbial
targets and resort to NETosis when their targets are “too large to

eat.” They also use NETosis to trap and kill fungi such as Candida

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