Have you ever sprained your ankle?
Did you notice redness and swelling near the injury? These symptoms indicate that your body is attempting to fight off infection.
The Immune System's Second Line of Defense
The little girl pictured below (Figure below) has a scraped knee. A scrape is a break in the skin that may let pathogens enter the body. If bacteria enter through the scrape, they could cause an infection. These bacteria would then face the body’s second line of defense. The second line of defense is also nonspecific, fighting many types of pathogens.
This little girl just got her first scraped knee. It doesn’t seem to hurt, but the break in her skin could let pathogens enter her body. That’s why scrapes should be kept clean and protected until they heal.
The body's second line of defense against pathogens includes the inflammatory response. If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. Inflammation is caused by chemicals that are released when skin or other tissues are damaged. The chemicals cause nearby blood vessels to dilate, or expand. This increases blood flow to the damaged area, which makes the area red and slightly warm. The chemicals also attract white blood cells called neutrophils to the wound and cause them to leak out of blood vessels into the damaged tissue.
White Blood Cells
What do these white blood cells do at the site of inflammation? The main role of white blood cells is to fight pathogens in the body. There are actually several different kinds of white blood cells. Some white blood cells have very specific functions. They attack only certain pathogens. Other white blood cells attack any pathogen they find. These white blood cells travel to areas of the body that are inflamed. They are called phagocytes, which means “eating cells.” Neutrophils are a type of phagocyte. In addition to pathogens, phagocytes “eat” dead cells. They surround the pathogens and destroy them. Sometimes it is said that the phagocyte engulfs the pathogen, and then destroys it. This process is called phagocytosis.
White blood cells also make chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6°F (37°C). Most bacteria and viruses that infect people reproduce fastest at this temperature. When the temperature is higher, the pathogens cannot reproduce as fast, so the body raises the temperature to kill them. A fever also causes the immune system to make more white blood cells. In these ways, a fever helps the body fight infection.
- If pathogens enter your body, inflammation occurs.
- White blood cells called phagocytes travel to areas of the body that are inflamed.
Use the resource below to answer the questions that follow.
- What is a macrophage? What does it do when it recognizes a "non-self" substance?
- What are cytokines? What message do they send to the rest of the body's cells?
- How do macrophages interact with T-cells? Where does this interaction occur?
- Describe inflammation.
- What is the main role of white blood cells?
- Describe phagocytosis.
- A fever is a sign of infection. Why might it be considered a good sign?