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 (
) 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.
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.
If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of 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. The chemicals also attract white blood cells 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 pathogens they find. These white blood cells travel to areas of the body that are inflamed. They are called
, which means “eating cells.” In addition to pathogens, phagocytes “eat” dead cells. They surround the pathogens and destroy them. This process is called
White blood cells also make chemicals that cause a fever. A
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.
: Higher-than-normal body temperature in response to infection.
: Response of the body to injury or infection that brings on redness, swelling, and pain.
: Type of white blood cell that ingests and destroys pathogens and dead cells.
: Engulfing of a large particle, such as a pathogen, by the cell.
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?
A fever is a sign of infection. Why might it be considered a good sign?