Immunity

Before pathogens can even reach the bloodstream, your body has several PHYSICAL and CHEMICAL barriers:

• Skin: tough outer layer of dead cells; pathogens can't penetrate intact skin.
• Mucus (in airways, nose): traps inhaled pathogens; tiny hairs (cilia) sweep them out.
• Stomach acid (HCl): kills most pathogens in food and drink.
• Tears, sweat, saliva: contain enzymes (lysozyme) that destroy bacteria.

When the skin is broken (a cut), the body forms a CLOT to seal the wound — and the immune system is alerted to deal with anything that may have gotten in.

These barriers are NON-SPECIFIC — they work against everything.

If pathogens get past the barriers, white blood cells (leucocytes) attack them. Two main types:

Phagocytes: 'eating' cells. They surround a pathogen with their cell membrane and engulf it into a vesicle, where enzymes digest it. Like Pac-Man for germs. Non-specific — eat anything foreign.

Lymphocytes: produce antibodies — Y-shaped proteins that bind to a specific pathogen. Each lymphocyte makes ONE kind of antibody. Antibodies work by:

• Tagging pathogens for destruction (other immune cells follow the tag).
• Clumping pathogens together so phagocytes can engulf many at once.
• Neutralising toxins.

After fighting an infection, some lymphocytes become MEMORY CELLS that stay in the blood for years. If the same pathogen invades again, memory cells multiply rapidly and produce LOTS of antibodies very quickly — usually before symptoms appear. This is IMMUNITY.

This is why you get chickenpox once and never again. The first infection trains memory cells; the second time, the immune system handles it before you notice.

A vaccine contains a weakened, dead or fragmented form of a pathogen (or sometimes genetic material that codes for parts of the pathogen — mRNA vaccines).

When injected:

1. The immune system treats it as a real infection.
2. Lymphocytes make antibodies against it.
3. MEMORY CELLS are produced.
4. If you encounter the real pathogen later, memory cells respond rapidly — usually preventing disease.

Vaccines have nearly ELIMINATED several diseases:

• Smallpox: eradicated worldwide (1980).
• Polio: eliminated from most countries; near global eradication.
• Measles, mumps, rubella: rare in countries with high vaccination rates.
• HPV: vaccinating teenagers nearly eliminates cervical cancer in the next generation.

Herd immunity: when ENOUGH people are vaccinated, the pathogen can't find a chain of susceptible people to keep spreading. Even those who can't be vaccinated (babies, immunocompromised) are protected. The threshold varies by disease:

• Measles: 95% vaccination needed.
• Polio: 80%.
• COVID-19: still being studied; complicated by variants.

If vaccination rates DROP, diseases that were nearly gone can come back. This has happened in several countries with declining MMR uptake → measles outbreaks.

MYP criterion D classic: 'Should vaccinations be mandatory by law?' Strong answers cover: individual rights vs collective benefit, herd immunity, the moral case for protecting the immunocompromised, the rare risk of vaccine side effects, and the role of education vs coercion.