The Immune System
What is the Immune System?
The immune system is the body's defence network against pathogens — disease-causing agents including bacteria, viruses, fungi, and parasites. It operates through multiple overlapping lines of defence, from physical barriers to targeted molecular responses.
A pathogen causes disease by:
- Reproducing rapidly inside the body
- Releasing toxins that damage cells and tissues
- Directly destroying host cells (as viruses do)
First Line of Defence: Physical and Chemical Barriers
The first line stops pathogens from entering the body at all. These defences are non-specific — they act against all pathogens equally.
- Skin — a waterproof, physical barrier; sebaceous glands produce oils that are mildly antimicrobial
- Mucus and cilia — mucus in the airways traps pathogens and particles; cilia (tiny hair-like structures) beat upward to move mucus away from the lungs
- Stomach acid — hydrochloric acid (pH 1–2) kills most pathogens swallowed with food
- Tears and saliva — contain lysozyme, an enzyme that breaks down bacterial cell walls
- Earwax — traps particles and has antimicrobial properties
Second Line of Defence: Non-Specific Immune Response
If pathogens breach the physical barriers, the non-specific immune response activates immediately.
Phagocytosis:
- Phagocytes (a type of white blood cell) are attracted to the site of infection
- They engulf the pathogen by surrounding it with their cell membrane
- The pathogen is enclosed in a vacuole and digested by enzymes
- The debris is expelled from the phagocyte
Inflammation:
- Blood vessels widen (vasodilation), increasing blood flow to the infected area
- This causes redness, warmth, and swelling
- More phagocytes are delivered to fight the infection
Fever:
- Raised body temperature slows pathogen reproduction
- Enhances the activity of immune cells
Third Line of Defence: Specific Immune Response
The specific immune response targets particular pathogens using lymphocytes (a type of white blood cell).
How it works:
- Pathogens carry unique surface proteins called antigens
- Lymphocytes recognise these specific antigens
- The correct lymphocytes multiply rapidly (clonal selection)
- B lymphocytes produce antibodies — proteins that bind specifically to the antigen
- Antibody-antigen binding marks pathogens for destruction or neutralises toxins
- T lymphocytes destroy infected body cells directly
- After infection, memory cells remain in the body
Memory Cells and Secondary Response
When the same pathogen is encountered again, memory cells:
- Recognise the antigen immediately
- Divide rapidly to produce large numbers of antibodies
- Mount a response so fast that symptoms may not even develop
This is why you do not usually get the same illness twice (e.g., chickenpox).
Vaccination
A vaccine triggers the specific immune response without causing disease. Vaccines contain:
- Dead or weakened pathogens
- Isolated antigens from pathogens
- mRNA instructions to make a specific antigen (newer technology)
After vaccination, memory cells are formed. If the real pathogen is encountered later, the immune response is rapid and powerful.
Herd immunity: when a high proportion of a population is vaccinated, even unvaccinated individuals are protected because the pathogen cannot spread easily.
Types of Immunity
| Type | How gained | Duration |
|---|---|---|
| Active (natural) | Infection by pathogen | Long-lasting |
| Active (artificial) | Vaccination | Long-lasting |
| Passive (natural) | Antibodies from mother via breast milk or placenta | Short-lived |
| Passive (artificial) | Antibody injection (antiserum) | Short-lived |
Passive immunity is immediate but does not produce memory cells, so it does not last.
Antibiotics and Antivirals
Antibiotics kill or inhibit bacteria by targeting structures unique to bacterial cells (cell walls, ribosomes). They do NOT work on viruses because viruses use the host cell's own machinery and lack these structures.
Antivirals interfere with viral replication inside host cells. They are harder to develop because viruses replicate inside human cells, making it difficult to target the virus without harming the host.
Overuse of antibiotics leads to antibiotic resistance — bacteria evolve to survive antibiotic treatment.
Key Terms
- Pathogen — a microorganism that causes disease
- Antigen — surface protein on a pathogen that triggers an immune response
- Antibody — protein produced by lymphocytes that binds to a specific antigen
- Phagocyte — white blood cell that engulfs and destroys pathogens
- Lymphocyte — white blood cell that produces antibodies or destroys infected cells
- Memory cell — long-lived lymphocyte that enables rapid response to future infection
- Vaccine — preparation that stimulates immunity without causing disease
Common Mistakes
- Confusing antigens (on pathogens) with antibodies (produced by lymphocytes)
- Thinking antibiotics cure viral infections — they only work on bacteria
- Saying passive immunity is long-lasting — it is short-lived because no memory cells are produced
- Confusing phagocytes (engulf pathogens) with lymphocytes (produce antibodies)
Tips and Tricks
- Remember: Antigens are on the pathogen; Antibodies are made by the body
- Antibody specificity is like a lock and key — one antibody fits only one antigen shape
- Active immunity = your body does the work (long-lasting); Passive = borrowed antibodies (short-lived)
- The secondary immune response is faster and stronger than the primary — this is the basis of vaccination