The big idea: If a pathogen gets past the skin and other barriers, the body's first responders are the cells of the innate immune system.
The key innate cells are phagocytes — white blood cells (leucocytes) that engulf and digest invading pathogens.
The innate response is non-specific (it attacks any pathogen), fast, and has no memory — it works the same way every time.
- Innate immune system
- The body's fast, non-specific defence that responds to any pathogen the same way, with no memory of past infections. Its main cells are phagocytes.
- Pathogen
- A microorganism that causes disease — for example a bacterium, virus or fungus.
- Leucocyte (white blood cell)
- A blood cell that defends the body against pathogens. Phagocytes and lymphocytes are both types of leucocyte.
- Phagocyte
- A type of leucocyte that engulfs and digests pathogens by phagocytosis. Macrophages and neutrophils are examples.
- Non-specific
- Acting against many different pathogens in the same way, rather than targeting just one type.
Innate = built in, ready straight away: The word innate means 'something you are born with'.
Your phagocytes are ready straight away — they do not need to 'learn' a pathogen first, so they act within minutes to hours.
This is the opposite of the adaptive system, which is slow and specific (you meet that in 3.7.4).
Phagocytes destroy pathogens by a process called phagocytosis — literally 'cell eating'.
There is no registered diagram for this process, so picture the steps as a sequence: the phagocyte recognises the pathogen, engulfs it, encloses it in a vacuole, and then digests it with enzymes.
- Phagocytosis
- The process in which a phagocyte engulfs a pathogen, encloses it in a vacuole, and digests it using enzymes.
- Vacuole
- A membrane-bound 'bubble' inside the phagocyte that holds the engulfed pathogen while it is broken down.
- Enzyme (digestive)
- A protein that speeds up the breakdown of the pathogen inside the vacuole, destroying it.
- Macrophage
- A large, long-lived phagocyte that engulfs pathogens and cell debris. (A neutrophil is a shorter-lived, fast-acting phagocyte.)
Phagocytosis — four steps to picture: 1. Recognise. The phagocyte detects the pathogen as foreign (non-self) — and it does this non-specifically, so it works on any pathogen.
2. Engulf. The phagocyte's cell membrane surrounds the pathogen and pulls it inside the cell.
3. Enclose. The pathogen is sealed inside a membrane-bound vacuole.
4. Digest. Enzymes are released into the vacuole and break the pathogen down, destroying it.
| Step | What the phagocyte does | Why it matters |
|---|---|---|
| 1. Recognise | Detects the pathogen as 'non-self' (foreign) | It does this non-specifically — to any pathogen, not just one type |
| 2. Engulf | Surrounds the pathogen and takes it inside the cell | The pathogen is pulled in by the cell membrane |
| 3. Enclose | Traps the pathogen in a membrane-bound vacuole | The vacuole keeps the pathogen sealed away inside the cell |
| 4. Digest | Enzymes break the pathogen down | The pathogen is destroyed and removed |
Why 'non-specific' is the word to remember: A phagocyte does not need to recognise one particular pathogen.
It engulfs anything identified as foreign — a bacterium, a virus-infected cell, or debris.
That is exactly why the innate response is described as non-specific, and why it can act immediately without 'learning' the invader first.
Innate (phagocytes)
- Non-specific — attacks any pathogen
- Fast — acts in minutes to hours
- No memory — same response every time
- Destroys pathogens by phagocytosis
Adaptive (lymphocytes)
- Specific — targets one pathogen / antigen
- Slow — takes days to build up
- Has memory — faster the second time
- Makes antibodies (covered in 3.7.4)
A memory hook: Phago = eat. A phagocyte is an 'eating cell' — it engulfs the pathogen, bags it in a vacuole, and digests it.
Innate = fast, non-specific, no memory. Adaptive = slow, specific, has memory.
| Feature | Innate immune system | Adaptive immune system |
|---|---|---|
| Cells involved | Phagocytes (e.g. macrophages, neutrophils) | Lymphocytes (B-cells and T-cells) |
| Specific to one pathogen? | No — non-specific (acts against any pathogen) | Yes — specific to one pathogen / antigen |
| Speed of response | Fast (acts within minutes to hours) | Slow (takes days to build up) |
| Memory of past infection? | No memory — same response every time | Has memory — faster, stronger second time |
| What it does | Engulfs and digests pathogens by phagocytosis | Makes antibodies / targets specific antigens |
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How this is tested: Paper 1A loves a quick Identify question: it lists several cell types and asks which one belongs to the innate immune system. The answer is a phagocyte (macrophage or neutrophil) — not a lymphocyte, B-cell or T-cell (those are adaptive).
Paper 2 can ask you to outline or describe how a phagocyte deals with a pathogen — give the steps of phagocytosis (engulf → enclose in a vacuole → digest with enzymes) and use the word non-specific.
Paper 1B often shows a table or graph of cell counts during an infection and asks you to read the data — the phagocyte count rises first because the innate response is the fast one.
IB-style question — identify the innate cell
A student lists four cells: a macrophage, a B-cell, a helper T-cell and a memory cell. State which cell belongs to the innate immune system, and give one reason for your choice. [2]
How to score both marks
- Pick the phagocyte. The macrophage is the cell of the innate immune system.
- Give a reason. A macrophage is a phagocyte: it engulfs and digests pathogens non-specifically, with no memory. The B-cell, helper T-cell and memory cell are all lymphocytes of the adaptive system. (Mark 1: macrophage. Mark 2: it is a phagocyte / acts non-specifically by phagocytosis, unlike the lymphocytes.)
Final answer
The macrophage — it is a phagocyte that engulfs and digests pathogens non-specifically; the B-cell, helper T-cell and memory cell are all adaptive lymphocytes.
✓ Why this scores full marks: It names the right cell (macrophage) and justifies it with the defining feature of the innate system — non-specific phagocytosis.
A common slip is choosing a lymphocyte (B-cell or T-cell): those are adaptive, not innate.
| Feature | Innate immune system | Adaptive immune system |
|---|---|---|
| Cells involved | Phagocytes (e.g. macrophages, neutrophils) | Lymphocytes (B-cells and T-cells) |
| Specific to one pathogen? | No — non-specific (acts against any pathogen) | Yes — specific to one pathogen / antigen |
| Speed of response | Fast (acts within minutes to hours) | Slow (takes days to build up) |
| Memory of past infection? | No memory — same response every time | Has memory — faster, stronger second time |
| What it does | Engulfs and digests pathogens by phagocytosis | Makes antibodies / targets specific antigens |