5.2 Pathogens and immunity

Why this matters

Throughout history, the biggest killers have been infectious diseases — plague, smallpox, cholera, tuberculosis, malaria. Today we control most of them in wealthy countries, but globally pathogens still kill millions every year (HIV, TB, malaria, plus emerging diseases like COVID-19). Understanding how pathogens infect, how the body fights back, and how vaccines work is some of the most important biology you learn at school. The four types of pathogen (bacteria, viruses, fungi, protists) all have different structures and life cycles. The body's defences are layered — physical barriers (skin, mucus, cilia), chemical barriers (stomach acid, lysozyme), and the immune system itself (phagocytes, B-lymphocytes, antibodies, memory cells, T-helper lymphocytes). Edexcel goes deeper on the cellular biology of the immune response than some boards: the named cell type for antibody production is the B-lymphocyte; antibodies are 'complementary in shape' to a specific antigen; memory cells are the basis of both natural immunity after infection AND vaccine-induced immunity. HIV is on the spec as an example of what happens when the immune system itself is attacked.

How to learn this topic

Build on what you already know

• 1BI0 1.1: cell structures — prokaryote vs eukaryote; viruses are not cells.
• 1BI0 1.2: blood contains white blood cells.
• KS3: germs spread between people; vaccines protect us.

1. Define pathogens and the four types — bacteria, viruses, fungi, protists.
2. Routes of transmission (air, water, direct contact, vectors).
3. Physical and chemical barriers (Edexcel marking phrase emphasis).
4. Phagocytes — non-specific engulfing of pathogens.
5. B-lymphocytes + antigen-antibody specificity — Edexcel core.
6. Memory cells and the secondary response (faster, larger).
7. Vaccination — antigens → antibodies → memory cells.
8. HIV — destroys T-helper lymphocytes, weakens immune system.

Key terms

pathogen

A microorganism that causes an infectious (communicable) disease. Four types: bacteria, viruses, fungi, protists. (Edexcel reward 'microorganism that causes disease'. Don't just say 'germ'.)

antigen

A molecule on the surface of a pathogen that the immune system recognises as foreign. Each pathogen carries unique antigens. (Antigens are on the PATHOGEN. Antibodies are made by the BODY. Don't muddle them.)

antibody

A Y-shaped protein produced by B-lymphocytes that is complementary in shape to one specific antigen, binds to it, and tags the pathogen for destruction. (Edexcel marking phrase: 'complementary to / specific to the antigen'. Use one of those exact words.)

B-lymphocyte

A type of white blood cell that recognises specific antigens and produces antibodies complementary to them. Some become memory cells after the infection. (Edexcel specifically expects 'B-lymphocyte' — not just 'lymphocyte' or 'white blood cell' — in 5- and 6-mark answers.)

T-helper lymphocyte

A type of white blood cell that coordinates the immune response by activating B-lymphocytes and other immune cells. The cell type that HIV destroys. (Only needed for HIV/AIDS questions — but get the name right when it comes up.)

phagocyte

A type of white blood cell that engulfs and digests pathogens (phagocytosis). A non-specific defence — works on any pathogen. (Phagocytes engulf. B-lymphocytes produce antibodies. Different jobs.)

memory cell

A long-lived B-lymphocyte that remains in the blood after an infection or vaccination. Responds rapidly to the same pathogen if it appears again, producing antibodies before symptoms develop. (Memory cells are key for second exposure AND vaccination — two different mark-scheme contexts.)

vaccine

A preparation containing antigens — from a dead, inactive or weakened pathogen — introduced into the body to trigger antibody production and memory cell formation, without causing disease. (Edexcel marking phrase: 'vaccine contains antigens'. State that explicitly, not just 'a vaccine has weak pathogens'.)

herd immunity

When a large enough proportion of the population is immune (through vaccination) that the pathogen cannot spread effectively, protecting unvaccinated individuals too. (Marking phrase: 'reduces the spread' or 'protects those who cannot be vaccinated'.)

HIV

A virus that infects and destroys T-helper lymphocytes, gradually weakening the immune system. Spread by exchange of body fluids; controlled but not cured by antiretroviral drugs. (Edexcel marking phrase: 'HIV destroys white blood cells / immune system is weakened'.)

AIDS

Acquired immunodeficiency syndrome — the late-stage condition in which an HIV-weakened immune system can no longer fight off other infections effectively. (HIV is the virus; AIDS is the syndrome it eventually causes. Don't use the words interchangeably.)

complementary shape

The shape of an antibody's binding site exactly matches the shape of one specific antigen — like a key fitting one lock. (Edexcel specifically reward the word 'complementary' in antigen-antibody questions.)

Notes

Pathogens — four types

A pathogen is a microorganism that causes an infectious disease:

| Type | Structure | Example |

|---|---|---|

| Bacteria | prokaryotic cells (no nucleus, have plasmids) | tuberculosis; cholera |

| Viruses | not cells — genetic material in a protein coat | measles; HIV; influenza |

| Fungi | eukaryotic, often multicellular | athlete's foot; rose black spot (plants) |

| Protists | eukaryotic, single-celled | malaria (Plasmodium, spread by mosquitoes) |

Pathogens spread by air (droplets — flu, COVID), water (cholera), direct contact (athlete's foot, STIs), vectors (mosquitoes spreading malaria) and contaminated food (salmonella).

Physical and chemical barriers (Edexcel marking phrases)

Before pathogens reach your blood, they have to get past several barriers. Edexcel reward these phrases verbatim:

• Mucus traps pathogens in the airways.
• Cilia waft mucus up the trachea — ciliated epithelial cells beat in unison to sweep mucus (with trapped pathogens) up to the throat to be swallowed.
• Hydrochloric acid in the stomach kills pathogens that are swallowed in mucus or food.
• These are physical and chemical barriers — physical (skin, mucus, cilia, nose hairs) and chemical (stomach acid, lysozyme in tears and saliva).

The immune response — what happens when pathogens get in

If pathogens breach the barriers, two layers of immune response kick in.

### Phagocytes — non-specific

Phagocytes are white blood cells that engulf pathogens (phagocytosis) and digest them with enzymes inside the cell. They work on any pathogen — non-specific, fast, no memory.

### B-lymphocytes — specific (Edexcel core content)

B-lymphocytes are the white blood cells responsible for producing antibodies. The Edexcel marking phrases describe the first exposure to a pathogen like this:

1. Antigens on the surface of the pathogen — each pathogen carries unique molecules (antigens) on its outer surface.
2. B-lymphocytes recognise the antigen — circulating B-lymphocytes have receptors that match particular antigen shapes.
3. Antibodies complementary to / specific to the antigen are produced — the activated B-lymphocyte multiplies and produces large numbers of Y-shaped antibody proteins whose binding sites are complementary in shape to that one antigen.
4. Antibodies bind to antigens on the pathogen — locking on and tagging the pathogen so phagocytes destroy it.

Key idea: each antibody is specific to one antigen — like a key fitting one lock. A different pathogen needs a different antibody.

### Memory cells — the second exposure

After the infection clears, some B-lymphocytes become memory cells that remain in the blood — sometimes for life. If the same pathogen ever returns:

1. B-lymphocytes produce antibodies specific to the antigen.
2. Memory cells remain in the blood after the first infection.
3. Memory cells respond faster on second exposure — producing antibodies much more rapidly and in larger amounts than the first time.
4. The pathogen is destroyed before symptoms appear.

This is immunity — you don't get the disease twice (for most pathogens).

Vaccination

A vaccine trains your immune system without giving you the disease. It works the same way as a natural first exposure, but uses a dead, inactive or weakened pathogen (or just antigens). The Edexcel marking phrases for vaccination:

1. The vaccine contains antigens — from a dead, inactive or weakened pathogen.
2. B-lymphocytes produce antibodies specific to those antigens.
3. Memory cells remain in the blood.
4. If the live pathogen later enters the body, the pathogen is destroyed before symptoms develop — because memory cells respond rapidly.

Vaccination gives active immunity — your own B-lymphocytes have been triggered to make antibodies and memory cells.

Herd immunity — when a high proportion of the population is vaccinated, the pathogen cannot spread effectively. Even unvaccinated people (e.g. infants, immunocompromised) are protected because they are unlikely to come into contact with an infectious person.

HIV — when the immune system itself is attacked

HIV (human immunodeficiency virus) is on the Edexcel spec because it shows what happens when the immune system breaks down. The Edexcel marking phrases:

1. HIV destroys white blood cells — specifically T-helper lymphocytes, which normally coordinate the immune response.
2. The immune system is weakened.
3. Fewer antibodies are produced because the coordination signal from T-helper cells is lost.
4. The body cannot fight off other infections effectively — leading to AIDS (acquired immunodeficiency syndrome), in which normally harmless infections become life-threatening.

HIV is spread by exchange of body fluids (unprotected sex, sharing needles, mother to baby). It can be controlled — not cured — with antiretroviral drugs that slow the virus's destruction of T-helper cells.

Putting it all together

For an Edexcel 6-mark answer on vaccination, examiners want to see the chain: vaccine contains antigens → B-lymphocytes recognise them → antibodies produced complementary to the antigens → memory cells remain in the blood → on second exposure, memory cells respond faster → pathogen destroyed before symptoms develop. Use the named cells (B-lymphocyte, memory cell) and the word 'complementary' — vague answers about 'the body fighting it off' lose marks even when broadly right.

Exam tips

• On any antibody question, use the word 'complementary' — Edexcel mark schemes reward 'antibodies complementary in shape to the antigen' over vague 'matches' or 'fits'.
• Name 'B-lymphocytes' specifically — not just 'lymphocytes' or 'white blood cells' — in extended-answer immune response questions.
• On vaccination questions, the four-part chain is: vaccine contains antigens → B-lymphocytes produce antibodies → memory cells remain in the blood → pathogen destroyed before symptoms develop. Hit all four.
• On second-exposure questions, the key contrast is SPEED and AMOUNT — memory cells produce antibodies faster and in larger quantities than the first response.
• For HIV, follow the chain: HIV destroys white blood cells (T-helper lymphocytes) → immune system weakened → fewer antibodies → body cannot fight off other infections effectively.
• Barriers — Edexcel like the phrases 'mucus traps pathogens', 'cilia waft mucus up the trachea', 'hydrochloric acid in the stomach kills pathogens'. Categorise them as 'physical and chemical barriers'.
• Don't confuse antigen with antibody. Memory aid: anti-GEN = on the GERM; anti-BODY = made by the BODY.

Mark-scheme phrasing

Common misconceptions

• —
• —
• —
• —
• —

Worked example

Question:

Answer:

Frequently asked questions