50 free Immune System trivia questions with answers — science & nature quiz, new questions added Aug 2026.
This immune system trivia quiz covers the network of cells and molecules that keeps you alive in a world full of pathogens. The easy questions are the ones any biology student knows: which white blood cell is most abundant, where T cells get their name, what antibodies are made by, which organ filters blood and recycles old red cells, and what a vaccine actually does. From there it moves into the two-tier design of innate and adaptive immunity, phagocytosis, the four signs of inflammation, the complement cascade, and how the body tells self from non-self. The hard end covers immunology proper: killer versus helper T cells and their CD8 and CD4 co-receptors, MHC class I and II, the five antibody isotypes, IgE and Type I hypersensitivity, natural killer cells and missing self, toll-like receptors, thymic involution, why HIV targets CD4 cells and how fever is set in the hypothalamus. There is history too: Jenner and James Phipps, Metchnikoff's starfish larvae, Ehrlich's magic bullet, von Behring and Kitasato, and the discovery of interferon. Every answer was checked against Wikipedia's articles on the immune system, its cells, organs and pioneers, and each question links to the page that establishes it. Fifty questions, free, with an explanation after every answer.
30 of 50 questions with answers and explanations. Play the quiz
Q 01The immune system has two major subsystems. The innate system gives a preconfigured response; what does the other, adaptive system do?
Learns to recognise molecules it has met before
Its immunological memory is the basis of vaccination.
Q 02Which type of white blood cell is the most abundant, making up 50-70% of leukocytes?
Neutrophils
Roughly 100 billion are produced each day and they live only 5 to 135 hours.
Q 03The process by which cells such as macrophages engulf and digest pathogens is called what?
Phagocytosis
The engulfed pathogen is trapped in a phagosome that fuses with a lysosome; it is probably the oldest form of host defence.
Q 04Which Russian zoologist discovered how cells engulf invaders, in 1882, by sticking citrus thorns into starfish larvae?
Élie Metchnikoff
Working in a private lab in Messina, Sicily, he watched unusual cells surround the thorns; he is called the father of innate immunity.
Q 05The 1908 Nobel Prize in Physiology or Medicine went jointly to the founder of cellular immunology and which German scientist, author of the side-chain theory?
Paul Ehrlich
Ehrlich also coined the "magic bullet" idea and discovered Salvarsan, the first drug aimed at a specific pathogen.
Q 06T cells get their name from which organ, where they mature?
Thymus
Developing T cells migrate there and are stimulated by thymosin.
Q 07The organ where T cells mature reaches its maximum size at puberty and then shrinks in a process called what?
Involution
It can weigh 40-50 grams at its peak.
Q 08Lymphocytes derive from haematopoietic stem cells in which tissue?
The bone marrow
Human marrow turns out about 500 billion blood cells a day.
Q 09B cells are responsible for which arm of adaptive immunity?
The humoral response
T cells handle cell-mediated immunity; activated B cells become plasma cells pumping out antibodies.
Q 10Killer T cells recognise antigens presented on which MHC molecules, and carry which co-receptor?
MHC I and CD8
Helper T cells, by contrast, read Class II MHC with their CD4 co-receptor.
Q 11Killer T cells release which pore-forming cytotoxin to punch holes in a target cell's membrane?
Perforin
Granulysin then enters and induces the target cell to undergo apoptosis.
Q 12Roughly how many MHC:antigen complexes must bind a helper T cell to activate it, compared with a single one for a killer T cell?
200-300
Helper cells also need longer engagement with an antigen-presenting cell.
Q 13What is the shape of a single antibody molecule?
Y-shaped
Its variable tips bind antigen; the tail, or Fc region, is far less variable.
Human antibodies come in how many classes, or isotypes?
Q 21What are the four classic symptoms of inflammation?
Redness, swelling, heat and pain
They are caused by increased blood flow, driven by eicosanoids and cytokines from injured cells.
Q 22Which class of eicosanoid produces fever and dilates blood vessels during inflammation?
Prostaglandins
Leukotrienes attract white blood cells; interleukins carry messages between them.
Q 23Fever results from a raised temperature set point in which part of the brain?
The hypothalamus
Hyperthermia, by contrast, is a rise above an unchanged set point and does not respond to antipyretics.
Five
IgA, IgD, IgE, IgG and IgM; IgG and IgA are further split into subclasses.
Q 15Which antibody class is transported across the placenta from mother to fetus?
IgG
Babies are born with their mother's range of antigen specificities; colostrum then supplies more antibodies to the gut.
Q 16Which antibody class mediates Type I hypersensitivity, the immediate reaction behind allergy and anaphylaxis?
IgE
It is normally the least abundant isotype and evolved to fight parasitic worms.
Q 17The study of antibodies began in 1890 when Emil von Behring and which Japanese bacteriologist described serum activity against diphtheria and tetanus toxins?
Kitasato Shibasaburō
They proposed the theory of humoral immunity.
Q 18Antigens were originally so named because they are what?
Antibody generators
They are non-self molecules that bind specific immune receptors and provoke a response.
Q 19Natural killer cells recognise and destroy compromised host cells by detecting what condition?
Missing self
Cells with low levels of MHC I, as in some viral infections, lose the brakes that killer-cell immunoglobulin receptors normally apply.
Q 20Natural killer cells make up roughly what share of circulating lymphocytes in humans?
5-20%
They are identified as CD56-positive and CD3-negative.
Q 24The complement system is a cascade of more than how many proteins that attacks foreign cell surfaces?
20
It is named for its ability to complement the killing of pathogens by antibodies, and ends in a membrane attack complex.
Q 25Coating a pathogen's surface with complement or antibody to mark it for destruction is called what?
Opsonisation
Complement fragments also attract immune cells and increase vascular permeability.
Q 26In 1891 Hans Buchner named the bactericidal property of blood "alexin", from a Greek word meaning what?
To ward off
Jules Bordet then split it into a heat-stable sensitiser and a heat-sensitive part that Ehrlich renamed complement in 1899.
Q 27Toll-like receptors, which detect pathogen-associated molecular patterns, were first discovered in which organism?
Drosophila fruit flies
Ten toll-like receptors have been described in humans.
Q 28Dendritic cells are named for their resemblance to what?
The branches of neurons
Found in skin, nose, lungs and gut, they present antigens to T cells, linking innate and adaptive immunity.
Q 29Which granulocyte, resident in connective tissue and mucous membranes, is most associated with allergy and anaphylaxis?
Mast cell
Basophils and eosinophils mainly defend against parasites and contribute to asthma.
Q 30Which organ acts as a blood filter, removes old red blood cells and holds a reserve of blood?
The spleen
It also recycles iron and is built like a very large lymph node.