- Define and explain The Body's Defences in your own words
- Use key terms such as antibody accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on The Body's Defences: discover skin, stomach acid, white blood cells and antibodies that fight infection.
Discover skin, stomach acid, white blood cells and antibodies that fight infection.
Key ideas
The immune response
The body's first defences are barriers: skin, mucus, and hydrochloric acid in the stomach. If pathogens get through, white blood cells called phagocytes engulf them, while lymphocytes produce specific antibodies. After the infection, memory lymphocytes remain, so a second encounter triggers a faster, stronger response — this is the basis of natural immunity.
Types of pathogen
Bacteria are single-celled organisms; some cause disease by releasing toxins. Viruses are far smaller and can only reproduce inside living cells, damaging them. Fungi such as athlete's foot thrive on skin, while protists such as the malaria parasite are spread by vectors like mosquitoes. Knowing the type matters because treatments differ: antibiotics kill bacteria but do nothing against viruses.
Key term — antibody: A Y-shaped protein made by white blood cells that locks onto a specific antigen on a pathogen, marking it for destruction.
Give two barriers that stop pathogens entering the body.
Any two of: intact skin; mucus trapping microbes; hydrochloric acid in the stomach killing microbes; hairs and mucus in the nose.
Answer: Any two of: intact skin; mucus trapping microbes; hydrochloric acid in the stomach killing microbes; hairs and mucus in the nose.
- Taking antibiotics for a cold or flu Colds and flu are caused by viruses, and antibiotics only kill bacteria. Using them unnecessarily also encourages antibiotic resistance.
- Thinking vaccines give you the disease Vaccines contain dead, weakened or partial pathogens that cannot cause the disease; any mild symptoms come from the immune response itself.
Practice
A phagocyte is a white blood cell that engulfs pathogens and digests them with enzymes, destroying the invaders.
Antibiotics target structures and processes found in bacterial cells, such as cell walls. Viruses lack these structures and reproduce inside human cells, so antibiotics cannot attack them.
The vaccine triggers an immune response that produces memory lymphocytes. If the real pathogen later invades, these memory cells mount a rapid response before the person becomes ill.
Bacteria – e.g. food poisoning or tuberculosis; viruses – e.g. flu or measles; fungi – e.g. athlete's foot; protists – e.g. malaria.
Quick check
Which of these best defines "antibody"?
Give three lifestyle choices that reduce the risk of heart disease.
- The Body's Defences: discover skin, stomach acid, white blood cells and antibodies that fight infection.
- The immune response: The body's first defences are barriers: skin, mucus, and hydrochloric acid in the stomach.
- immunity: The ability to resist a particular disease, usually because memory cells can mount a fast response if the pathogen returns.
- Watch out for: taking antibiotics for a cold or flu