Middle School Life Science: Diseases and the Body's Defenses - First Two Lines of Defense — Flashcards

The first line of defense against infection are the surface barriers that prevent the entry of pathogens into the body. The second line of defense are the non-specific phagocytes and other internal mechanisms that comprise innate immunity. 

If bacteria enter the skin through a splinter or other wound, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries and is part of the body's second line of defense. It occurs due to chemicals that are released when tissue is damaged. The chemicals cause nearby blood vessels to dilate, increasing blood flow to the area. The chemicals also attract white blood cells to the area. The white blood cells leak out of blood vessels and into the damaged tissue.    
The white blood cells that go to a site of inflammation and leak into damaged tissue are called phagocytes. They start “eating” pathogens and dead cells by engulfing and destroying them. This process is called phagocytosis.    
Phagocytes also release chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 37 °C (98.6 °F). Most bacteria and viruses that infect people reproduce quickly at this temperature. When the temperature rises higher, the pathogens can't reproduce as quickly. Therefore, a fever helps to limit the infection. A fever also causes the immune system to make more white blood cells to fight the infection.