Immunity

Step 1. Pathogen enters the body and releases chemicals (or its presence is detected by other immune signals).

Step 2. Phagocytes (a type of WBC) MIGRATE to the site.

Step 3. Phagocyte recognises the pathogen as foreign and EXTENDS its membrane around it.

Step 4. Pathogen is fully ENGULFED into a vesicle inside the cell.

Step 5. LYSOSOMES (full of enzymes) fuse with the vesicle. Enzymes DIGEST the pathogen.

Step 6. Some pathogen fragments are displayed on the phagocyte surface — alerts lymphocytes.

Why this is non-specific. Phagocytes don't 'know' the pathogen — they just detect anything that's foreign. Works on viruses, bacteria, dead cells, etc.

Worked qualitative. Why is pus yellow-white?

• Pus = a mixture of dead phagocytes, dead pathogens, fluid, and tissue debris.
• Phagocytes die after engulfing pathogens (lysosomes break themselves down too).
• The colour comes from the breakdown products and proteins.

Cambridge tip. Memorise the two-stage process: ENGULF → DIGEST. Don't say phagocytes 'fight' or 'attack' — those are too vague.

Antigens. Markers (proteins or other molecules) on the SURFACE of a pathogen. Each pathogen has its own unique antigens — like a barcode.

Lymphocytes. White blood cells with receptors that match ONE specific antigen. The body has millions of different lymphocytes, each waiting to find its match.

The response:

1. A lymphocyte's receptor BINDS to a matching antigen on a pathogen.
2. The lymphocyte ACTIVATES → divides rapidly (clonal expansion) into many identical cells.
3. Most clones become PLASMA CELLS that pump out millions of identical ANTIBODIES.
4. Some clones become MEMORY CELLS that survive for years/lifetime.

What antibodies do:

• AGGLUTINATE: bind to multiple pathogens, clumping them together → easier for phagocytes to engulf.
• MARK: tag pathogens so phagocytes recognise them as foreign.
• NEUTRALISE: bind to bacterial TOXINS → block their harmful effects.
• PREVENT BINDING: stop pathogens attaching to body cells (e.g. blocking flu virus from entering cells).

Specificity.

• One antibody = one antigen.
• An anti-flu antibody won't work on COVID — different antigen.
• That's why you can catch a cold many times: each strain has slightly different antigens.

Worked qualitative. Why does a cold last about a week?

• Day 1-3: pathogen multiplies fast; symptoms peak; lymphocytes still scaling up.
• Day 4-7: antibody production hits high level → pathogens cleared → symptoms ease.
• Memory cells produced → faster response next time you meet the same strain.

Cambridge tip. Cambridge marks 'specific antibody' for each antigen. Don't say antibodies 'fight germs' generally — link each antibody to ONE antigen.

The principle. Show the immune system the antigens of a pathogen WITHOUT actually infecting you.

What vaccines contain.

• DEAD pathogens (inactivated by heat or chemicals).
• WEAKENED (attenuated) pathogens — alive but harmless.
• Just the ANTIGENS (purified proteins).
• mRNA coding for an antigen (e.g. some COVID-19 vaccines).
• All trigger an immune response without causing disease.

What happens after a vaccine:

1. Antigens enter the bloodstream.
2. Lymphocytes recognise them → activate → divide.
3. Some become PLASMA CELLS (short-term antibody production).
4. CRUCIAL: some become MEMORY CELLS — long-lived.
5. After ~2 weeks, the body has antibodies AND memory cells specific to that pathogen.

On future exposure:

• Real pathogen invades.
• Memory cells recognise it INSTANTLY.
• Antibody production is huge and fast.
• Pathogen cleared BEFORE symptoms appear → immunity.

Boosters.

• Some vaccines need REPEAT DOSES to maintain memory.
• Memory cells decline over years; boosters refresh them.

Herd immunity.

• If most people in a community are vaccinated, the pathogen can't spread easily.
• Protects vulnerable people who can't be vaccinated (babies, immunocompromised).
• Threshold varies — measles needs ~95% to stop transmission.

Worked qualitative. Why does vaccination wipe out diseases like smallpox?

• Smallpox is exclusively human (no animal reservoir).
• Vaccination created herd immunity worldwide.
• Without susceptible hosts, the virus had no one to infect.
• Declared eradicated in 1980 — the only human disease yet eradicated by vaccination.

Cambridge tip. When asked HOW a vaccine works, your answer MUST mention MEMORY CELLS. Without memory cells, the long-term immunity wouldn't exist.

Active immunity.

• The body MAKES its own antibodies after meeting an antigen.
• Sub-types:
  • NATURAL active: from infection (catching the disease).
  • ARTIFICIAL active: from vaccination.
• Memory cells produced → LONG-LASTING (often lifetime).

Passive immunity.

• Antibodies are RECEIVED from another organism — body doesn't make them.
• Sub-types:
  • NATURAL passive: maternal antibodies through PLACENTA + BREAST MILK.
  • ARTIFICIAL passive: anti-venom injection, anti-snake-bite serum.
• NO memory cells made → SHORT-LASTING (weeks to months).
• Useful for IMMEDIATE protection (e.g. snake bite, rabies exposure).

Comparison.

Feature	Active	Passive
Source of antibodies	Self-made	Received
Speed	Slow (1-2 weeks)	Immediate
Duration	Long (years/life)	Short (weeks/months)
Memory cells	Yes	No

Worked qualitative. Why are babies often immune to diseases their mothers have had?

• During pregnancy, antibodies cross the PLACENTA → baby's bloodstream.
• After birth, antibodies in BREAST MILK enter baby's gut.
• This is NATURAL PASSIVE immunity.
• Lasts a few months — until baby's own immune system matures.
• That's why vaccination schedules start at 2 months — baby's system can now make memory cells.

Cambridge tip. Cambridge often gives a scenario asking "is this active or passive?" The trick: ask "did the person make their own antibodies?". If yes → active. If received → passive.