A communicable disease (also called an infectious disease) is one caused by a PATHOGEN — a microorganism that infects and harms the host.
Communicable diseases can be SPREAD from one organism to another — either directly or through a vector (a carrier organism like a mosquito).
There are four main types of pathogen:
BACTERIA — single-celled prokaryotes. Reproduce rapidly by binary fission inside the body. Cause disease mainly by producing TOXINS. Examples: Salmonella, Gonorrhoea, Tuberculosis.
VIRUSES — not true cells. Much smaller than bacteria. Enter host cells and use the cell's machinery to replicate, destroying the host cell in the process. Examples: Measles, HIV, Influenza, TMV.
FUNGI — eukaryotic organisms. Grow on or in host tissue. Examples: Athlete's foot, Rose black spot.
CONTAMINATED WATER — drinking or bathing in water containing pathogens. Example: cholera (bacteria), typhoid.
DIRECT CONTACT — skin contact, sexual contact, touching contaminated surfaces. Examples: athlete's foot (skin contact), gonorrhoea (sexual contact), rose black spot (plant contact).
VECTORS — an organism that carries the pathogen but doesn't cause the disease itself. Example: Anopheles mosquito carries Plasmodium (malaria) and injects it when it bites.
BLOOD CONTACT — sharing needles, transfusions. Example: HIV, Hepatitis B.
The Body's Physical and Chemical Barriers
The body has several lines of defence against pathogens — the first line stops pathogens from entering at all.
SKIN — a tough, continuous physical barrier. As long as the skin is unbroken, most pathogens cannot pass through it. The skin also produces slightly acidic secretions that inhibit bacterial growth.
MUCUS — goblet cells in the lining of the airways, nose and throat produce sticky mucus. Pathogens and particles breathed in get trapped in this mucus before they can reach the lungs.
CILIA — tiny hair-like structures on the cells lining the trachea and bronchi. They beat rhythmically, sweeping the mucus (with trapped pathogens) upwards towards the throat where it is swallowed. The stomach acid then kills any pathogens.
STOMACH ACID — hydrochloric acid (HCl) in the stomach is very acidic (pH ~2). Most pathogens that are swallowed are killed here before they can cause infection.
The Immune System — White Blood Cells
If pathogens get past the physical barriers, the IMMUNE SYSTEM takes over.
PHAGOCYTES:
These are white blood cells that carry out PHAGOCYTOSIS — they identify and engulf (eat) pathogens.
The cell membrane wraps around the pathogen and pulls it inside the cell.
Enzymes inside the phagocyte then digest and destroy it.
Phagocytes are NON-SPECIFIC — they attack any pathogen they encounter without needing to learn its identity first.
LYMPHOCYTES:
These white blood cells produce ANTIBODIES — proteins with a specific shape that binds to ANTIGENS on the surface of a particular pathogen.
Each lymphocyte makes ONE type of antibody for ONE specific antigen.
Antigens are molecules on the surface of pathogens that the immune system identifies as foreign.
Antibodies can neutralise pathogens, mark them for destruction, or cause them to clump together.
After an infection, MEMORY CELLS remain in the blood for years or for life.
If the same pathogen invades again, memory cells rapidly produce large quantities of antibodies — destroying the pathogen before it causes disease. This is IMMUNITY.
⚠️ Common Mistake
Antibiotics kill BACTERIA only — they have absolutely NO effect on viruses. Never say 'take antibiotics for a virus'. Flu is caused by a virus — antibiotics will not help. Also: phagocytes ENGULF pathogens. Lymphocytes PRODUCE ANTIBODIES. These are different white blood cells with different jobs.
📌 Key Note
Physical barriers: skin, mucus, cilia, stomach acid — first line of defence. Phagocytes: engulf and destroy (non-specific). Lymphocytes: make specific antibodies. Memory cells: rapid response on re-infection = immunity.
🎯 Matching Activity — Match the Defence to How it Works
Match each defence mechanism to its correct description. — drag the symbols on the right to match the component names on the left.
Skin
Drop here
Cilia
Drop here
Stomach acid
Drop here
Phagocytes
Drop here
Lymphocytes
Drop here
Memory cells
Drop here
Sweep mucus with trapped pathogens upwards towards the throat
Physical barrier — prevents pathogens entering the body as long as it is unbroken
Remain after infection — allow rapid antibody response if same pathogen returns
pH ~2 — kills most pathogens that are swallowed
Engulf and digest pathogens — non-specific immune defence
Produce specific antibodies that bind to pathogen antigens
⭐ Higher Tier Only
The secondary immune response is faster and larger than the primary — memory B-cells rapidly differentiate into plasma cells producing large quantities of specific antibodies. This is the basis of vaccination and acquired immunity. Students should be able to explain how memory cells allow rapid response on re-exposure to a specific pathogen.
🎯 Test Yourself
Question 1 of 4
1. What is the difference between a phagocyte and a lymphocyte?
2. Why are memory cells important for immunity?
3. How does the Anopheles mosquito spread malaria?
4. Why does the body produce mucus in the airways?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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