The Innate Immune System
Objective 10
Compare the lines of defense associated with innate immunity. Identify the key components of the innate immune system and which ones belong to which line of defense. Describe phagocytosis, fever, and inflammation. Describe the components of the complement system and its role in immunity.
The chemical and physical barriers of the first line of defense include: skin, mucus, mucus-coated hairs, cilia, the lacrimal apparatus, vaginal secretions, sebaceous secretions, and gastric fluids. It also includes reactions such as vomiting, defecation, salivation, urination, perspiration, coughing, sneezing and blinking. Each of these barriers or responses protects the various external openings of the body.
The second line of defense includes a large number of internal defenses. As part of this line, the body produces natural antimicrobial substances like interferons, complement, and iron-binding proteins to decrease microbial growth.
Non-specific phagocytes and natural killer cells are part of this second line.
There are two groups of phagocytes in the second line of defense, neutrophils and macrophages. Neutrophils are released in large numbers, very quickly, but their lifespan is relatively short. The bone marrow releases monocytes that enlarge and develop into macrophages as they migrate to the site of infection or injury. These are called wandering macrophages. Other macrophages, called fixed macrophages, may occupy specific tissues and have specific names like Kupffer cells in the liver or microglial cells in the central nervous system.
In a similar way, phagocytic cells are attracted to the chemical signals given off by foreign microbes or cells and will move towards them in a process called chemotaxis. Once the membrane of the phagocytic cells comes in contact with the thing to be eaten it will stick to it.
After this adherence, the phagocytic cell will bring the thing in through endocytosis. Digestive enzymes and other chemicals inside phagocytic cells help to break the cell into pieces, effectively digesting and killing it. The final step that a phagocytic cell can perform is to express a piece of the destroyed cell on its surface to alert other immune cells of threat it is responding to. Antigen presentation is an important connection between the innate and adaptive immune responses.
Fever & Inflammation
When the hypothalamus shifts the body’s thermostat to increase temperature, we call it fever. Fever is part of our non-specific internal defenses. Fever is often thought of as a bad thing, and from a patient-comfort point of view, that probably is correct. But, to fight off disease, fever is a good thing. An increase in body temperature speeds up chemical reactions, increases antimicrobial activity, and sequesters (hides) nutrients essential for bacterial growth. Remember those boring chemistry labs in high school? What did you do if you wanted to speed up a reaction?
Inflammation is a big part of our non-specif