Every pathogen carries its own antigens, each with a specific shape. Lymphocytes make antibodies whose shape fits only that antigen. The first time takes days; the second time, memory cells make it fast enough that you never feel ill. Vaccination gives you that second time without the first. This whole lesson is Extended content.
Extended: active immunity is defence against a pathogen by antibody production in the body. Each pathogen has its own antigens with specific shapes, and specific antibodies are proteins with complementary shapes that bind to them, leading to direct destruction of the pathogen or marking it for destruction by phagocytes. Active immunity follows infection or vaccination and produces memory cells for long-term protection. Passive immunity is short-term: antibodies come from another individual (across the placenta, in breast milk) and no memory cells are made.
Fig. 1: an antibody binds only to an antigen whose shape is complementary to its binding site
Section 01
Antigens and complementary antibodies.
Extended. There are three kinds of pathogen in the sim, each with its own antigen shape: red cones, blue cubes and yellow balls. Pick an antibody, press Release antibodies, and watch which pathogens it binds to. Then watch the phagocyte.
Antigens: each pathogen has its own antigens, molecules on its surface with specific shapes.
Antibodies: proteins made by lymphocytes (a type of white blood cell). Specific antibodies have complementary shapes which fit specific antigens, so an antibody made against one pathogen does not bind to another.
What happens next: binding leads to direct destruction of the pathogen, or marks the pathogen (and clumps pathogens together) so that phagocytes engulf and destroy them.
Active immunity: defence against a pathogen by antibody production in the body itself.
Examiner trap
Say the antibody has a complementary shape to the antigen, not "the same shape". And antibodies are made by lymphocytes; phagocytes engulf pathogens but do not make antibodies.
Section 02
Infection, vaccination and memory.
Extended. Press Pathogen enters and follow the graph: antibody level (yellow) against the pathogens (green), with a dashed line for the level that stops the pathogen. Then let it enter a second time. Reset and try Vaccinate first, and then Breast milk antibodies.
First infection: only a few lymphocytes have the right shape, so it takes days to make enough antibodies. The pathogen multiplies meanwhile and you become ill. Memory cells are made.
Second infection: memory cells recognise the antigen at once; antibodies are made faster and in much larger amounts, and the pathogen is destroyed before you feel ill.
Vaccination: (a) weakened pathogens or their antigens are put into the body; (b) the antigens stimulate an immune response by lymphocytes, which produce antibodies; (c) memory cells are produced that give long-term immunity.
Active immunity is gained after an infection by a pathogen or by vaccination.
Passive immunity: a short-term defence by antibodies acquired from another individual, including across the placenta and in breast milk. Memory cells are not produced, so protection fades as the antibodies are broken down.
Breast-feeding passes antibodies to the infant, protecting it while its own immune system develops.
Examiner trap
Passive immunity gives no memory cells. Do not say breast milk "gives the baby memory cells" or long-term immunity; the antibodies work at once but only for a short time.
Section 03
Vaccination protects the population.
Extended. Set the percentage vaccinated with the slider and press Start the outbreak. Blue people are vaccinated, red are ill and green have recovered. Compare 30% with 80% and read the number of unvaccinated people protected.
Each vaccinated person has memory cells, so they do not become ill and cannot pass the pathogen on.
When enough people are vaccinated, an infected person rarely meets anyone who can catch the disease, so chains of transmission break and the outbreak dies out.
This also protects people who cannot be vaccinated, such as very young babies or people who are already ill.
Mass vaccination wiped out smallpox worldwide and has made diseases such as polio and measles rare in many countries.
Examiner trap
Herd protection depends on the proportion of the population vaccinated. If too few are vaccinated the disease still spreads through the unvaccinated people.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
Extended: in the antibodies sim, the lymphocyte is making antibody A, which fits cones. What happens when it meets a pathogen with blue cube antigens?
Quick check+10 XP
Extended: in the antibodies sim, which cell engulfs the pathogens once antibodies have bound to them?
Quick check+10 XP
Extended: in the vaccination sim, the pathogen enters a second time after a first infection. How does the antibody curve differ?
Quick check+10 XP
Extended: after breast milk antibodies are given in the vaccination sim, what does the memory cells readout show?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Extended: which statement describes antibodies?
Exam style+20 XP
Extended: why is a vaccine against measles no protection against chickenpox?
Exam style+20 XP
Extended: a vaccinated person meets the pathogen and does not become ill. What is the main reason?
Exam style+20 XP
Extended: which gives a newborn baby passive immunity?
Skill unlocked
Antibodies and vaccination, mastered.
This skill is now recorded on your profile. Keep the chain going.