Introduction to Body Defences

In our daily lives, we are constantly surrounded by pathogens (tiny organisms like bacteria and viruses that cause disease). If our bodies were completely open to the environment, we would be sick all the time! Fortunately, the human body has a brilliant multi-layered defence system designed to keep these invaders out or destroy them if they manage to get inside. Think of your body as a high-security fortress; in this chapter, we will look at the walls, the guards, and the specialized response teams that keep you safe.

The Body's First Line of Defence

Before a pathogen can make you ill, it has to get into your body. We have several mechanical and chemical barriers that act as our primary shields.

Mechanical Barriers

These are physical structures that block pathogens from entering.

  • Skin: This is your primary surface defence. It covers almost your entire body, providing a tough, waterproof layer that is very difficult for pathogens to penetrate unless it is cut or grazed.
  • Hairs in the nose: These act as a physical filter. When you breathe in, larger particles and dust (which often carry pathogens) get trapped in the hairs before they can reach your lungs.

Chemical Barriers

These use chemicals to trap or kill pathogens before they can do harm.

  • Mucus: This is a sticky substance produced by the lining of your airways (the trachea and bronchi). It acts like flypaper, trapping pathogens that you breathe in. (Quick link: Remember from the breathing chapter that ciliated cells then sweep this mucus away!)
  • Stomach Acid: If you swallow pathogens (perhaps on your food), the hydrochloric acid in your stomach is strong enough to kill most of them.

The Internal Defence: White Blood Cells

If a pathogen manages to bypass the skin or stomach acid, it enters the bloodstream. This is where your White Blood Cells take over. There are two main types you need to know:

1. Phagocytes: These cells "eat" pathogens. They identify a pathogen, surround it, and engulf it. Once the pathogen is inside the phagocyte, it is digested and destroyed. This process is called phagocytosis.

2. Lymphocytes: These are more specialized. They produce antibodies, which are protein molecules that have a specific shape to fit onto the surface of a pathogen and help destroy it.

Key Takeaway: Mechanical barriers (skin/hair) and chemical barriers (mucus/acid) stop pathogens from getting in. White blood cells deal with any that manage to break through.

Advanced Immunity (Supplement Only)

Note: This section is for students taking the Extended/Supplement curriculum.

Antigens and Antibodies

Every pathogen has unique molecules on its surface called antigens. Each antigen has a specific shape, like a biological "fingerprint."

Lymphocytes recognize these antigens. When they find a foreign one, they produce antibodies. An antibody is a protein with a complementary shape that fits onto the specific antigen of the pathogen. Once attached, antibodies can:

  • Clump pathogens together so they can't move or reproduce.
  • Mark the pathogens so that phagocytes can find and destroy them more easily.

Active Immunity and Memory Cells

Active immunity is when your body produces its own antibodies. This can happen after you catch a disease or after a vaccination.

When you encounter a pathogen for the first time, your lymphocytes work hard to create the right antibody. After the "battle" is over, your body keeps special cells called memory cells. If the same pathogen enters your body again, these memory cells recognize the antigens immediately and produce massive amounts of the correct antibody very quickly. You usually destroy the pathogen before you even feel sick!

Passive Immunity

Passive immunity is a short-term defence where you are given antibodies from another source (like from a mother to her baby through breast milk). Because your own body didn't make them, you don't make memory cells. Once those antibodies are used up or broken down, the protection is gone.

Cholera: A Case Study

Cholera is a disease caused by a bacterium. It is a great example of how pathogens can cause harm. The cholera bacterium produces a toxin (a poison). This toxin causes chloride ions to move out of the cells and into the small intestine. This lowers the water potential in the gut, causing water to move out of the blood and into the intestine by osmosis. This leads to intense diarrhoea and severe dehydration.

Controlling the Spread of Disease

Preventing the spread of pathogens is just as important as the body's internal defences. Here are the key ways we control the spread:

  • Clean Water Supply: Ensuring water is treated and free from pathogens like cholera.
  • Hygienic Food Preparation: Keeping raw and cooked foods separate, cooking meat thoroughly to kill bacteria, and washing hands before handling food.
  • Good Personal Hygiene: Regular handwashing with soap removes pathogens picked up from surfaces. Using tissues when sneezing/coughing prevents pathogens from becoming airborne.
  • Waste Disposal: Disposing of rubbish properly prevents flies and rats (which can carry diseases) from gathering near human homes.

Did you know? Even though we didn't always know about "germs," ancient civilizations often practiced forms of social distancing or used clean water sources because they noticed a link between dirt and disease!

Quick Review Tips

Don't worry if the names of the cells get confusing! Use these simple tricks to remember them:

  • Phagocytes are like Pac-Man — they Patrol and eat things.
  • Lymphocytes produce Lethal antibodies (and Long-term memory cells).
  • Active immunity = Your body did the work. Passive immunity = Someone Passed you the antibodies.

Common Mistake to Avoid: Many students think antibodies "eat" pathogens. They don't! Only phagocytes "eat" (engulf) them. Antibodies simply bind to them to help the process along.