The body's first defences stop pathogens getting in: skin is a barrier, the nose has hairs and mucus, the trachea has cilia that sweep mucus up, and stomach acid kills many microbes. If pathogens get in, white blood cells attack them. Phagocytes engulf and digest pathogens. Lymphocytes make antibodies that lock onto specific antigens on the pathogen, and antitoxins that neutralise toxins.
After an infection, memory cells remain, so a second infection is met with a faster, larger antibody response: you are immune. A vaccine contains dead or weakened pathogens or their antigens, so the body makes memory cells without getting ill. When most of a population is vaccinated, the pathogen struggles to spread; this is herd immunity.
Worked example
Why do you rarely catch chickenpox twice?
- The first infection makes lymphocytes produce antibodies.
- Memory cells stay in the blood.
Answer:Next time they make antibodies quickly, before you feel ill.
Key words
- antibody
- protein that binds a specific antigen
- antigen
- a molecule on a pathogen's surface
- phagocyte
- white blood cell that engulfs pathogens
- vaccine
- harmless form of a pathogen that triggers immunity
New questions every time.
NGSS-referenced: HS-LS1-2