IB Biology HLDisease & ImmunityPaper 1 & 2~9 min read
The Immune System
You have two immune systems, not one. The first reacts to anything foreign, straight away, and never learns. The second takes days to get going the first time, but it remembers. Almost every immunity question comes down to telling those two apart.
📘 What you need to know
An antigen is a molecule that can trigger an immune response. All cells carry antigens on their surface membranes.
Your own cells display self antigens. Anything foreign displays non–self antigens, and that is what triggers a response.
The innate system responds to any non–self item: bacteria, fungi, viruses, protists, pollen, dust.
You are born with the innate response and it does not change during your lifetime. It is non–specific.
Phagocytes are part of the innate response — they engulf and digest anything showing non–self antigens.
The adaptive system responds to specific non–self antigens, leading to antibody production and memory cells.
On a second encounter the adaptive response is much faster and makes many more antibodies, so the pathogen is destroyed before symptoms appear.
The adaptive system develops with age. Young babies have no adaptive immunity yet.
Self and non–self
Before either system can act, your body has to answer one question: is this thing mine? It answers it by reading the molecules on the surface of whatever it has bumped into.
Definition
An antigen is a molecule that can trigger an immune response.
Every cell has antigens on its cell surface membrane. Your own cells carry self antigens, so they are left alone. A foreign cell carries non–self antigens, so it sets off a response.
This is why allergies happen. Pollen grains and other allergens carry molecules the innate system reads as non–self. There is no pathogen and no infection, but the response fires anyway, and the symptoms of the allergy are the result.
The two systems side by side
The shield is the same for every attacker. The lock and key only fits one. Keep that picture in your head and the comparison questions get easy.
The innate immune system
The innate system recognises anything that enters the body and is non–self. It does not care what it is. Bacteria, fungi, viruses, protists, pollen grains and dust all get the same treatment, because they all display non–self antigens.
You are born with the ability to mount this response, and it does not change during your lifetime.
The action of phagocytes is part of it. A phagocyte will engulf and digest any item displaying non–self antigens.
It is described as non–specific, because it happens in response to any non–self antigen rather than one particular type.
“Non–specific” does not mean weak or sloppy. It means the response is the same whatever the invader is. That is a strength, because the innate system starts working immediately while the adaptive system is still identifying the enemy.
The adaptive immune system
The adaptive system responds to the presence of specific non–self antigens — for example, the antigens of one particular type of pathogen.
The first time it meets a new antigen, a sequence of events runs that eventually produces antibodies and leaves memory cells in the blood. The next time the same antigen appears, that sequence runs much more quickly and produces many more antibodies, so the pathogen is destroyed before any symptoms develop.
The adaptive system changes over a lifetime as you are exposed to different antigens.
This growing record of past pathogens is called immunological memory.
Young babies have no adaptive immunity at first. It develops with age.
Vaccination uses this system deliberately: it introduces the immune system to a new pathogen so the response on real exposure is already fast.
Adaptive responses are also called specific immune responses.
On a first exposure you usually feel ill, because the adaptive response is still being built. On a second exposure it fires straight away and you may notice nothing at all.
The comparison table to learn
Feature
Innate
Adaptive
Triggered by
Any non–self antigen
One specific non–self antigen
Also called
Non–specific response
Specific response
Present from
Birth
Develops with age as you meet antigens
Changes over a lifetime?
No
Yes — this is immunological memory
Speed
Immediate, every time
Slow on first exposure, very fast on repeat
Main cells
Phagocytes
Lymphocytes (T cells and B cells)
Produces antibodies?
No
Yes, along with memory cells
🧠
Innate = Inborn. Adaptive = Adjusts.
Innate is what you were born with and it never changes. Adaptive adjusts every time you meet something new.
Worked examples
WE 1
Distinguish between innate and adaptive immunity
Distinguish between the innate and adaptive immune systems. (3 marks)
Point 1: what triggers each
The innate system responds to any non–self antigen, so it is non–specific. The adaptive system responds to one specific antigen.
Point 2: change over time
The innate response is present from birth and does not change during a lifetime. The adaptive system develops with age as different antigens are encountered.
Point 3: memory
Only the adaptive system produces antibodies and memory cells, so only it responds faster on a second exposure.
Non–specific and fixed vs specific and improving“distinguish” means both sides in every point — write innate does X, adaptive does Y
WE 2
Explain how the body tells self from non–self
Explain how the immune system distinguishes between the body’s own cells and foreign cells. (2 marks)
Point 1: the markers
All cells display molecules called antigens on their cell surface membranes.
Point 2: the recognition
The body’s own cells carry self antigens and are not attacked. Foreign cells carry non–self antigens, which trigger an immune response.
Self antigens are ignored, non–self antigens trigger a responsesay antigen, not “marker” — the technical word is what earns the mark
WE 3
Apply it: why babies get ill more often
Suggest why young children tend to catch more infections than adults. (2 marks)
Point 1: what they are missing
Adaptive immunity develops with age. A young child has met fewer antigens, so has far fewer memory cells.
Point 2: the consequence
Most infections are a first exposure for them, so a slow primary response has to run each time and symptoms develop. An adult often has memory cells already and clears the pathogen before symptoms appear.
Fewer memory cells means more first exposuresdo not say their immune system is “weaker” — their innate system works perfectly well
💡 Exam tips
Learn the antigen definition exactly: a molecule that can trigger an immune response.
Use the pairs the mark scheme uses: innate / non–specific and adaptive / specific.
Say the innate response does not change during a lifetime. That phrasing is often the mark.
Link phagocytes to innate and lymphocytes to adaptive.
If a question mentions allergy, the answer involves non–self antigens on an allergen such as pollen.
Remember adaptive immunity develops with age — useful for any question about babies or vaccination schedules.
⚠ Common mistakes
Saying the innate system is weaker. It is broad and immediate, which is a different thing.
Saying antigens are only found on pathogens. Every cell has them, including yours.
Confusing antigen and antibody. The antigen triggers the response; the antibody is made in response.
Claiming the innate system makes memory cells. Only the adaptive system does.
Saying babies have no immune system. They have a full innate system; it is the adaptive part that is still developing.
Treating allergy as an infection. There is no pathogen — the response is triggered by a harmless allergen.
Up next: White Blood Cells. We have named phagocytes and lymphocytes without saying what they look like or where they come from. That is the next page.
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