The Immune System and Vaccination

White blood cells (leucocytes) are the key players in the immune response. They are produced in the bone marrow and circulate in the blood and lymph.

There are three main ways white blood cells defend against pathogens:

a) Phagocytosis

Phagocytes (a type of white blood cell) engulf and destroy pathogens:

1. The phagocyte detects the pathogen (recognises it as foreign)
2. The phagocyte engulfs (surrounds) the pathogen by extending its cytoplasm around it
3. The pathogen is enclosed in a vacuole inside the phagocyte
4. Enzymes inside the phagocyte are released into the vacuole, digesting and destroying the pathogen

Phagocytosis is a non-specific response — phagocytes attack any foreign organism.

b) Antibody Production

Lymphocytes (another type of white blood cell) produce antibodies — special proteins that target specific pathogens.

1. Each pathogen has unique antigens (proteins) on its surface
2. Lymphocytes recognise these foreign antigens
3. The lymphocyte produces antibodies that are complementary in shape to the pathogen's antigens — like a lock and key
4. Antibodies bind to the antigens, which can:
   • Clump pathogens together (agglutination) — making it easier for phagocytes to engulf them
   • Neutralise toxins produced by pathogens
   • Mark pathogens for destruction by other immune cells
5. After the infection, some lymphocytes remain as memory cells — if the same pathogen invades again, antibodies are produced much faster and in greater quantities (secondary immune response)

Key concept: Antibodies are specific — each type of antibody only works against one type of antigen. Different pathogens need different antibodies.

c) Antitoxin Production

Some lymphocytes produce antitoxins — molecules that neutralise (counteract) the toxins released by certain bacteria.

• Antitoxins bind to toxins and make them harmless
• This helps reduce the symptoms of disease caused by bacterial toxins

Vaccination (immunisation) is a way to provide artificial immunity against a specific disease without causing the disease itself.

How Vaccines Work

1. A vaccine contains a small quantity of dead or inactive forms of a pathogen (or parts of the pathogen's antigens)
2. These do not cause the disease but still carry the pathogen's antigens
3. The immune system recognises the foreign antigens and triggers an immune response
4. Lymphocytes produce antibodies specific to that pathogen
5. Memory cells are produced and remain in the body for years (or a lifetime)
6. If the real pathogen enters the body later, memory cells recognise it immediately and produce antibodies rapidly and in large numbers — the infection is destroyed before symptoms develop

Primary vs Secondary Immune Response

Herd immunity occurs when a large proportion of a population is vaccinated, reducing the spread of the disease.

• If most people are immune, the pathogen finds it difficult to spread from person to person
• This protects people who cannot be vaccinated (e.g., very young babies, people with weakened immune systems, people who are allergic to vaccine components)
• For herd immunity to work, a high percentage of the population must be vaccinated (typically 85–95%, depending on the disease)
• If vaccination rates drop, herd immunity breaks down and outbreaks can occur

Important: Antibiotics do NOT work against viruses. Viruses live inside host cells, so antibiotics cannot reach them without destroying the host's own cells.

The overuse and misuse of antibiotics has led to the emergence of antibiotic-resistant bacteria:

1. Random mutations in bacteria may make some resistant to an antibiotic
2. When antibiotics are used, non-resistant bacteria are killed, but resistant bacteria survive
3. Resistant bacteria reproduce rapidly, passing on the resistance gene
4. Over time, the entire population may become resistant
5. This creates "superbugs" like MRSA (methicillin-resistant Staphylococcus aureus)

How to Reduce Antibiotic Resistance

• Only prescribe antibiotics when necessary (not for viral infections)
• Always complete the full course of antibiotics
• Don't use antibiotics in animal farming unnecessarily
• Develop new antibiotics to replace those becoming ineffective
• Practice good hygiene in hospitals to prevent spread of resistant bacteria

Question: Explain why a person who has been vaccinated against measles is unlikely to catch measles if exposed to the virus. (4 marks)

Solution:

The measles vaccine contains dead or inactive measles virus (or its antigens). When injected, the immune system recognises the foreign antigens and produces antibodies specific to the measles virus. Memory lymphocytes are also produced and remain in the body long-term.

If the person is later exposed to the real measles virus, the memory cells recognise the antigens immediately and produce the correct antibodies rapidly and in large quantities. The virus is destroyed before it can multiply and cause symptoms. This is the secondary immune response, which is much faster and stronger than the primary response.

• Vaccines don't contain the "live disease" — always say dead, inactive, or weakened pathogens (or isolated antigens).
• Always link immunity to memory cells and the rapid secondary response.
• When explaining antibiotic resistance, use the word "mutation" — resistance arises by random genetic mutation, NOT because bacteria "learn" to resist antibiotics.
• Don't confuse antibodies (proteins that fight pathogens) with antibiotics (drugs that kill bacteria).

• White blood cells defend the body through phagocytosis (engulfing pathogens), antibody production (targeting specific antigens), and antitoxin production (neutralising toxins).
• Vaccination introduces dead/inactive pathogens to stimulate the immune system, producing memory cells for long-term immunity.
• Herd immunity protects those who cannot be vaccinated by reducing pathogen spread in the population.
• Antibiotics treat bacterial infections but NOT viral infections; overuse leads to antibiotic resistance.