Biology 43, 44, 45

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Chapter 43 - Immune System

- Pathogen - Agent that can cause diseases
We have dedicated cells, immune cells, that specifically interact with and destroy pathogens. Innate immunity - defense that is active immediately upon infection. Innate immunity responds the same way whether or not the pathogen has been encountered before. Very fast. ex: skin, shell > barrier to entry by microbes, pathogen lining of digestive system and airway. Body must detect foreign particles within the body - differentiate between self and non self. It detects non self by molecular recognition. Receptor molecule bind specifically to molecule of pathogen.

Adaptive Immunity - Only in vertebrates (animals with spinal cord) It makes a vast number of receptors, and each receptor recognizes a feature typically found on a pathogen. Adaptive cells are very specific. Acquired immunity > memory cells will remember pathogen from first infection. (Adaptive immunity has a great memory, if you get a certain cold once it will remember it and so if it comes again it will squash it right away).

See Figure 6

Innate immunity “armor” or “knights”
In all animals and plants
Ex: Insects - exoskeletons, chitin - barrier. In their intestines they have chitin that protects them from infection. They have an enzyme in their intestines called lysozyme. This enzyme breaks down bacterial cell wall, protects their digestive system. Within their hemolymph (their blood) they have hemocytes that help ingest and digest pathogens. They also secrete chemicals to entrap pathogens and block them from growing and they have peptides to inactivate. On their hemocytes they have toll receptors that recognize pathogens and start and immune response. Vertebrates

Barrier defense: We have epithelial cells that are tightly packed, blocking entry of any pathogens. This includes our skin, mucous membrane of the lung and digestive track. In these linings (specifically the lungs) we have cells that secrete mucus (thick fluid)that enhances defense by trapping microbes. We also have in cells that are found in the air pathway, that sweep the mucus and trap the microbes and will move the microbes outwards so that they cant move into the lungs. So the pathogens cant move into the lungs. Saliva and tears provide a washing mechanism that inhibits colonization by fungi and bacteria. We have lysozyme in tears and saliva and mucus that also destroys the cell walls. We have a very acidic stomach that can kill most pathogens. Cellular Innate Behavior: These are phagocytic cells that detect pathogens using several receptors. Toll like receptors (TLR), they are on phagocytic cells that will bind fragments of molecules characteristic of a set of pathogens. For ex:

TLR3 that binds to dsRNA viruses.
TLR4 will bind to lipopolysaccharide, cell wall of bacteria. After detecting the pathogen, the phagocytic cell engulfs and traps the pathogen in a vacuole. This vacuole will fuse with the lysosome and get destruction of pathogen.

Phagocytic cells in mammals (WBC)
Neutrophils - circulate the body in blood and attracted to infected tissues. Macrophages - large phagocytic cells some migrate and some are localized. Example: spleen, macrophages that kill pathogens from lymph. Dendritic cells populate tissues that contact the environment (skin), stimulating adaptive immune system against pathogens they encounter. Eosinophils - they are found on mucousal surfaces. Reduced phagocytic activity but they play a role in defending against multicellular invaders. Example: parasitic worms, they’ll discharge destructive enzymes. Natural killer T cells - circulate the blood and detect the abnormal array of surface proteins characteristic and viruses, infected cells and cancer cells. They don’t engulf but they’ll release chemicals that lead to cell death.

Antimicrobial peptides and proteins: When a pathogen is detected, they’ll trigger a...
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