Welcome to Your Body’s Defence Force!
Have you ever wondered why you don't get sick every single time you touch a dirty door handle or cough in a crowded room? It’s because your body is like a high-tech fortress, equipped with walls, traps, and a highly trained "security team" known as the immune system. In these notes, we will explore how your body keeps pathogens (the "bad guys") out and how it remembers them if they ever try to attack again.
1. The First Line of Defence: Barriers
Before a pathogen can make you ill, it has to get inside your body. Your body uses physical barriers to block them and chemical defences to kill them on contact.
Physical Barriers (The Walls)
Skin: Your skin is a thick, waterproof layer that covers almost your entire body. It acts as a continuous barrier that most pathogens simply cannot get through unless you have a cut or a graze.
Mucus: This is a sticky substance produced by the lining of your nose and airways (the breathing system). It works like flypaper, trapping dust and pathogens before they can reach your lungs.
Cilia: These are tiny, hair-like structures found on the cells lining your airway. They move in a waving motion to sweep the mucus (and the trapped pathogens) up to the back of your throat so you can swallow it into your stomach.
Chemical Defences (The Traps)
Lysozymes: These are special enzymes found in your tears, saliva, and mucus. Think of them as "chemical scissors" that react with and break down the cell walls of bacteria, killing them before they can enter the body.
Hydrochloric Acid: Your stomach is full of this strong acid. Most pathogens that you swallow in food or drink are destroyed by the very low \(pH\) of the acid in your stomach.
Quick Summary: Physical barriers (skin, mucus, cilia) block or trap pathogens, while chemical defences (lysozymes, acid) destroy them.
2. The Specific Immune System
If a pathogen manages to get past your barriers and into your blood, your specific immune system takes over. This system is "specific" because it creates a unique response for every different type of pathogen.
Antigens and Antibodies
Every pathogen has unique molecules on its surface called antigens. You can think of an antigen like a "Name Badge" or a "Uniform" that tells your body the cell is an invader.
When your white blood cells, called lymphocytes, come across a foreign antigen, they start to produce antibodies.
Key Points about Antibodies:
1. They are proteins with a specific shape.
2. They "lock onto" the antigens on the pathogen, marking them for destruction or clumping them together.
3. They are specific—an antibody made to fight the flu virus will not work against the bacteria that causes a sore throat.
Memory Lymphocytes and the Secondary Response
The first time you meet a new pathogen, it takes a few days for your lymphocytes to "figure out" the right antibody to make. This is why you feel ill.
However, after the infection is gone, your body keeps special cells called memory lymphocytes.
The Secondary Response: If the same pathogen enters your body again, these memory cells recognise it immediately. They produce massive amounts of antibodies so quickly that the pathogen is destroyed before you even feel any symptoms. You are now immune!
Common Mistake to Avoid: Don't confuse antigens with antibodies. Remember: Anti-Gens are on the "Germs," and Anti-Bodies are made by "your Body."
3. Immunisation (Vaccination)
Immunisation is a way of making you immune to a disease without you having to get sick first. It involves vaccination.
How it Works
1. A vaccine is injected into the body. It contains a dead, inactive, or weakened form of the pathogen.
2. The vaccine still has the antigens on its surface, but it cannot cause the disease.
3. Your lymphocytes treat the vaccine like a real threat and produce antibodies and memory lymphocytes.
4. If you ever meet the "real" live pathogen later, your memory cells trigger a fast secondary response to kill it.
Advantages and Disadvantages (Biology Only)
Why do we use immunisation?
Advantages:
- It protects individuals from dangerous diseases (like polio or measles).
- It can lead to herd immunity: if most people are vaccinated, the disease cannot spread easily, which protects people who cannot be vaccinated (like very small babies or very ill people).
- Some diseases can be completely wiped out (eradicated).
Disadvantages:
- Some people may have a mild reaction to the vaccine (like a sore arm or a fever).
- In very rare cases, people can have a serious allergic reaction.
- It doesn't always give 100% protection (sometimes you need "booster" shots).
Did you know? The word "vaccination" comes from the Latin word vacca, which means cow! This is because the very first vaccine was developed using the cowpox virus to protect people against smallpox.
Check Your Understanding
Can you explain...?
1. How cilia and mucus work together to protect the lungs?
2. Why a person only gets chickenpox once, but can get different types of colds?
3. The role of memory lymphocytes in the secondary immune response?
4. How a vaccine "tricks" the body into becoming immune?
Note: For more information on how we test the medicines used for immunisation, see the chapter on "Developing new medicines and monoclonal antibodies." For details on the pathogens themselves, see "Pathogens and the spread of infection."