Phagocytes must reach infected tissues and identify microbes before they can respond effectively. This lesson explains how circulating leukocytes leave the bloodstream and how phagocytes recognize pathogens using opsonins and pattern recognition receptors.
Some phagocytes, including neutrophils and monocytes, circulate in the blood. To reach microbes in infected tissues, they must pass through the walls of small blood vessels. This movement is called extravasation, or diapedesis.
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What are extravasation and diapedesis?
They are names for the process by which leukocytes leave the bloodstream by passing through vessel walls, particularly the thin walls of capillaries, to enter tissues.
What are PAMPs?
Pathogen-associated molecular patterns are microbial structures shared by broad groups of microbes, such as peptidoglycan, flagellin, LPS, lipopeptides, and viral nucleic acids.
What do pattern recognition receptors do?
PRRs detect PAMPs and trigger cellular signals that help phagocytes recognize microbes and initiate responses such as cytokine production and enhanced killing.
How does a phagosome differ from a lysosome?
A phagosome is a membrane-bound compartment containing newly engulfed material; a lysosome is an enzyme-containing organelle that can fuse with it.
What happens during a respiratory burst?
A phagocyte sharply increases oxygen uptake and uses the oxygen to generate antimicrobial reactive oxygen species.
Quiz questions
How does a phagocyte enclose a pathogen during phagocytosis?Answer: Pseudopods extend around it and enclose it in a membrane-bound phagosome.
What cellular event forms a phagolysosome?Answer: A lysosome fuses with a phagosome.
What is the approximate pH inside an acidified phagosome?Answer: About 4–5.
Which reaction produces peroxynitrite in phagocytes?Answer: Nitric oxide reacts with superoxide.
How does antigen presentation by macrophages and dendritic cells support immunity?Answer: They display selected pathogen antigens at the cell surface, helping stimulate adaptive immunity.
Based on Microbiology by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.