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Innate immunity is the first line of defense against pathogens. Innate immunity includes physical barriers such as the skin, and cellular responses involving the recognition of self and nonself molecules. Recognition of nonself molecules by white blood cells leads to coordinated responses such as the production of defensive proteins and inflammation.
learning outcomes
You should be able to:
Summarize the roles of physical barriers, chemical defenses, and cellular defenses in innate immunity.
Explain how the innate defense system defends against a pathogen.
List the steps in the inflammatory response.
A pathogenic bacterium lands on your skin. Outline the innate defenses that come into play.
The physical barrier of skin prevents infection. Mucus traps and removes bacteria so they cannot infect. Lysozyme hydrolyzes bacterial cell walls. Defensins insert into the bacterial cell membranes, rendering them leaky. Once below the skin, the bacteria come in contact with complement proteins, which provoke phagocytosis, and phagocytes that ingest and destroy the bacteria.
Your immune system does not mount an innate defense response to a speck of dust that lands on your skin, but it may mount an innate response to a bacterium. Why?
The bacterium has an arrangement of atoms on its molecules called PAMPs. These are recognized by receptors on the human body cells, and they initiate a response. Dust particles lack PAMPs.
When a splinter of wood permeates the skin, the skin swells up. Outline the events by which this occurs.
The splinter initiates the inflammatory response. See Figure 41.5.
Often the innate immune system, with its nonspecific defenses, is adequate to prevent or fight off a pathogenic infection. But in many cases this system works together with adaptive immunity, which detects and responds to specific pathogens. We will now turn to the development and functioning of adaptive immunity.