Immunology: How the Body Defends Itself
CollegeThe immune system runs a two-tier defence: a fast, general-purpose innate response ready from birth, and a slower but exquisitely specific adaptive response that learns, remembers, and gets stronger every time it meets the same threat.
Innate immunity: fast and general
The first line of defence is physical and chemical: skin, mucus, lysozyme in tears and saliva, and stomach acid. If a pathogen breaches these barriers, phagocytes (neutrophils, macrophages) recognise general molecular patterns shared by whole classes of pathogens — via pattern recognition receptors such as Toll-like receptors — and engulf and destroy invaders. The complement system is a cascade of blood proteins that can coat pathogens for easier phagocytosis (opsonisation) or punch holes directly in microbial membranes (the membrane attack complex). Damaged or infected tissue triggers inflammation: histamine widens blood vessels, bringing more immune cells and fluid to the site (redness, heat, swelling).
Adaptive immunity: slow, specific, and remembers
Adaptive immunity has two coordinated arms:
- Humoral immunity (B cells) — each B cell displays a unique antibody shape on its surface. When one happens to bind an antigen, that B cell is activated and undergoes clonal selection: it proliferates into a clone of plasma cells that mass-produce that specific antibody, plus long-lived memory B cells.
- Cell-mediated immunity (T cells) — helper T cells (CD4⁺) recognise antigen fragments presented on MHC class II (found on antigen-presenting cells) and coordinate the response, activating both B cells and cytotoxic T cells. Cytotoxic T cells (CD8⁺) recognise antigen on MHC class I, found on nearly all body cells, and directly kill infected or cancerous cells.
The primary response to a first exposure is slow (days) because only a few matching cells exist at first and must proliferate. The secondary response, driven by pre-existing memory cells, is much faster and stronger — often clearing a pathogen before symptoms appear. This is the biological basis of both natural immunity after infection and of vaccination.
Why the second response is so much faster
The first time a pathogen appears, the immune system must find it the hard way: among millions of B and T cells each carrying a randomly generated, unique receptor, only a tiny handful happen to fit the new antigen well. Those few must be found, activated, and expanded by repeated division before enough specific antibody or killer cells exist to win — which takes days, during which you feel ill. A vaccine, or a first true infection, leaves behind a standing army of memory cells — already numerous, already primed, and quicker to reactivate. On re-exposure, the immune system starts not from one-in-a-million odds but from thousands of matching, ready cells, so the response arrives faster and stronger than the pathogen can multiply — often eliminating it with no symptoms at all.
MHC I is on almost every cell and signals "look what's inside me" to cytotoxic T cells — the body's internal quality check. MHC II is only on specialised antigen-presenting cells and signals "look what I found outside" to helper T cells — the body's intelligence briefing. One is a self-check; the other is a report.
Worked example: reading an antibody titre graph, and herd immunity
Part A. A blood test shows antibody concentration ("titre") of 20 units after a first vaccine dose, rising to 20,000 units within days of a booster dose weeks later.
- Identify the responses. The first, low, slow-rising level is the primary response. The booster's sharp, high rise is the secondary response, driven by memory cells already in place.
- Quantify the difference. Fold increase = 20 000 ÷ 20 = 1000-fold higher titre, reached in far less time — direct evidence of immunological memory.
Part B — herd immunity threshold. Measles has a basic reproduction number R₀ ≈ 15 (each case infects ~15 others in a fully susceptible population).
- Recall the formula. Herd immunity threshold = 1 − 1/R₀.
- Insert the value. 1 − 1/15 = 1 − 0.067 = 0.933.
- Interpret. About 93% of the population must be immune before the chain of transmission reliably breaks — explaining why measles vaccination coverage targets are set so high compared with less contagious diseases.
Test yourself
Q1 Distinguish the roles of MHC class I and MHC class II in antigen presentation, and which T cell type responds to each.
MHC class I is expressed on almost all nucleated body cells and displays fragments of proteins made inside the cell; it is monitored by cytotoxic (CD8⁺) T cells, which kill the cell if the displayed fragment is foreign (e.g. viral). MHC class II is expressed mainly on specialised antigen-presenting cells (macrophages, dendritic cells, B cells) and displays fragments of material engulfed from outside the cell; it is recognised by helper (CD4⁺) T cells, which coordinate the wider immune response rather than killing directly.
Q2 Explain, in terms of cell numbers and kinetics, why the secondary immune response is faster and stronger than the primary response.
The primary response must first locate the very rare naive B or T cells whose randomly generated receptor happens to match the new antigen, then clonally expand them over several days before enough effector cells exist to control the infection. The secondary response instead starts from a much larger pool of memory cells generated during the first exposure — these are already numerous, already have a proven matching receptor, and require less activation, so they proliferate into effector cells (plasma cells, cytotoxic T cells) far faster and to a greater final magnitude.
Q3 Using the herd immunity formula 1 − 1/R₀, calculate the threshold for a pathogen with R₀ = 4, and explain what the result means practically.
Threshold = 1 − 1/4 = 1 − 0.25 = 0.75, or 75%. This means at least three-quarters of the population must be immune (via vaccination or prior infection) for the average infected person to transmit to fewer than one new person, causing outbreaks to shrink rather than grow — herd immunity protects even the unvaccinated minority by starving the chain of transmission.
Q4 Explain why an attenuated (weakened) or mRNA vaccine can produce immunological memory without causing the disease itself.
Both strategies present the immune system with recognisable antigens — either from a live-but-weakened pathogen unable to cause serious disease, or from a harmless mRNA instruction that makes the body's own cells briefly produce one viral protein (such as a spike protein) — without the pathogen's disease-causing machinery being present. B and T cells still detect these antigens as foreign, undergo clonal selection, and generate memory cells, exactly as a real infection would, but without the pathogen ever having the ability to cause the actual illness.
Q5 Type 1 diabetes results from the immune system destroying the pancreas's own insulin-producing cells. Explain this in terms of self-tolerance.
Normally, the immune system undergoes a screening process (largely in early T-cell development) that eliminates or suppresses immune cells that react strongly to the body's own ("self") molecules — this is self-tolerance. In an autoimmune disease like type 1 diabetes, this tolerance breaks down: cytotoxic T cells mistakenly treat the pancreas's insulin-producing beta cells as foreign and destroy them, exactly as they would destroy a virus-infected cell. The result is a loss of insulin production, showing that autoimmune disease is fundamentally a failure of the immune system's normally reliable self/non-self discrimination.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The process, step by step
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1A titre graph shows a small slow first peak and a large fast second peak. Explain.
- First exposure needs clonal selection, so it is slow and small
- Memory cells make the second response fast and strong
Example 2A disease needs 80% herd immunity. In a class of 30, how many must be immune?
- 0.80 x 30 = 24
- Round up to protect the group
Now you try
Work each one out first, then tap to reveal the worked answer.