FIGURE 1.

The hierarchy of neutrophil granule exocytosis. (A) Neutrophils contain various subsets of cytoplasmic granules that are classified based on their content as follows: secretory vesicles, tertiary or gelatinase granules, specific or secondary granules, and azurophilic or primary granules. They also contain at least two sets of additional organelles with secretory capacity: late endosomes and recycling endosomes. Membrane and cargo proteins are indicated in black, while secretory effectors are in red. Notice that Rab27a confers a subpopulation of azurophilic granules the category of secretory organelle. (B) The granules are exocytosed in a hierarchical order with the secretory vesicles being the most readily exocytosed, followed by the tertiary and specific granules. The hierarchy of exocytosis of these granules also correlates with the functions exhibited by their cargo proteins. (C) The azurophilic granules, with the most toxic cargoes, are ideally exocytosed in response to the strongest stimuli found at the site of infection where the concentration of pathogens is the highest