3.6.4Additional lessons
The Immune System — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 73 of 142
Part 1 of 5Antigens and Self versus Non-self
Antigen
- A molecule that can trigger an immune response, usually a protein or carbohydrate on a cell surface.
Self antigens
- All the body's own cells carry antigens on their surface membranes, and the immune system normally ignores them.
Non-self antigens
- Bacteria, viruses, fungi, protists and pollen carry antigens that the body recognises as foreign.
Recognition
- The immune system distinguishes self from non-self, and failure of this recognition can lead to autoimmune disease.
Part 2 of 5The Innate Immune System
Present at birth
- Its defences are already in place and do not need a previous encounter with the pathogen.
Non-specific
- It responds to any non-self material in a similar way instead of targeting one antigen.
Phagocytes
- These cells engulf and digest any item that displays non-self antigens.
Fixed response
- The response is the same on every exposure and does not improve with experience.
Part 3 of 5The Adaptive Immune System
Specific
- It responds to particular antigens and produces defences matched to that one pathogen.
Lymphocytes
- B cells and T cells carry out the adaptive response.
Memory
- Memory cells stay in the blood, so a second encounter with the same antigen leads to a faster, larger response.
Develops with age
- Because exposure builds a record of antigens met, immunity to different pathogens increases through life.
| Feature | Innate | Adaptive |
|---|---|---|
| Speed | Fast | Slower on first exposure |
| Specificity | Non-specific | Specific |
| Memory | None | Yes |
| Main cells | Phagocytes | B and T lymphocytes |
Part 4 of 5Primary and Secondary Response
1
On first exposure, B cells and T cells specific to the antigen must be selected and multiplied, so antibodies appear slowly.
2
Some of the activated lymphocytes become memory cells that remain after the infection ends.
3
On a later exposure, memory cells respond rapidly and divide to form many plasma cells.
4
The secondary response makes more antibodies sooner, so the pathogen is destroyed before symptoms develop.
Part 5 of 5Exam Checklist & Tips
3.6.4 – Key Points for the Exam
- Antigen A molecule that triggers an immune response.
- Innate immunity The immediate, non-specific defences present from birth.
- Adaptive immunity The specific response that produces memory.
- Memory cell A long-lived lymphocyte that allows a faster response on re-exposure.
Common Mistakes
- Saying the innate system has memory.
- Writing that antibodies are antigens; they are proteins made against antigens.
- Stating that a person is immune after one infection from any pathogen, when immunity is specific to that antigen.
- Thinking that self cells have no antigens.
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