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48 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.
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Q 01Unlike the innate system, what does the adaptive immune system do?
Learns to recognise molecules it has met before
The adaptive system learns to recognise molecules it has met before, building immunological memory through B and T cells; that memory is the basis of vaccination.
Q 02Which type of white blood cell is the most abundant, making up 50-70% of leukocytes?
Neutrophils
Neutrophils make up 50–70% of circulating leukocytes; roughly 100 billion are produced each day and they live only about 5 to 135 hours.
Q 03The process by which cells such as macrophages engulf and digest pathogens is called what?
Phagocytosis
Phagocytosis traps the engulfed pathogen in a phagosome that fuses with a lysosome; it is probably the oldest form of host defence, present even in amoebae.
Q 04Which Russian zoologist discovered how cells engulf invaders, in 1882, by sticking citrus thorns into starfish larvae?
Élie Metchnikoff
Élie Metchnikoff, working in a private lab in Messina, Sicily, watched mobile cells surround the thorns in starfish larvae; he is called the father of innate immunity.
Q 05T cells get their name from which organ, where they mature?
Thymus
T stands for thymus-derived: immature T cells migrate to the thymus, where they undergo positive and negative selection and are stimulated by thymosin.
Q 06The organ where T cells mature reaches its maximum size at puberty and then shrinks in a process called what?
Involution
Thymic involution begins after puberty, when the gland is at its largest; over the following decades functional tissue is replaced by fat and output of new T cells falls.
Q 07Lymphocytes derive from haematopoietic stem cells in which tissue?
The bone marrow
Bone marrow houses the haematopoietic stem cells that give rise to all lymphocytes; human marrow turns out about 500 billion blood cells a day.
Q 08B cells are responsible for which arm of adaptive immunity?
The humoral response
B cells drive the humoral response: activated B cells become plasma cells pumping antibodies into body fluids, while T cells handle cell-mediated immunity.
Q 09Killer T cells recognise antigens presented on which MHC molecules, and carry which co-receptor?
MHC I and CD8
Cytotoxic T cells carry the CD8 co-receptor and read peptides on MHC I, which almost all nucleated cells display; helper T cells read class II with CD4.
Q 10Killer T cells release which pore-forming cytotoxin to punch holes in a target cell's membrane?
Perforin
Perforin polymerises into pores in the target membrane; granzymes then enter through them and trigger apoptosis in the infected or cancerous cell.
Q 11Roughly 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 12What is the shape of a single antibody molecule?
Y-shaped
An antibody is Y-shaped: two variable arm tips bind antigen, while the tail, or Fc region, is far less variable and engages other immune cells.
Q 13Human antibodies come in how many classes, or isotypes?
Five
There are five classes: IgA, IgD, IgE, IgG and IgM; IgG and IgA are further split into subclasses.
Q 21Which class of eicosanoid produces fever and dilates blood vessels during inflammation?
Prostaglandins
Prostaglandins such as PGE2 cause fever and dilate blood vessels, which is why aspirin, a prostaglandin blocker, lowers both; leukotrienes attract white blood cells instead.
Q 22Fever results from a raised temperature set point in which part of the brain?
The hypothalamus
Pyrogens raise the hypothalamus's temperature set point, producing fever; hyperthermia, by contrast, is a rise above an unchanged set point and does not respond to antipyretics.
Q 23The complement system is a cascade of more than how many proteins that attacks foreign cell surfaces?
20
Q 14Which antibody class is transported across the placenta from mother to fetus?
IgG
IgG is actively carried across the placenta by the neonatal Fc receptor, giving babies their mother's range of antigen specificities; colostrum then supplies more antibodies to the gut.
Q 15Which antibody class mediates Type I hypersensitivity, the immediate reaction behind allergy and anaphylaxis?
IgE
IgE mediates the immediate Type I reaction behind allergy and anaphylaxis; it is normally the least abundant isotype and evolved to fight parasitic worms.
Q 16The study of antibodies began in 1890 when Emil von Behring and which Japanese bacteriologist described serum activity against diphtheria and tetanus toxins?
Kitasato Shibasaburō
Kitasato Shibasaburō and von Behring showed in 1890 that serum from immunised animals neutralised diphtheria and tetanus toxins, founding the theory of humoral immunity.
Q 17Antigens 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 18Natural killer cells recognise and destroy compromised host cells by detecting what condition?
Missing self
The 'missing self' hypothesis: cells with low MHC I, as in some viral infections or tumours, lose the brakes that killer-cell immunoglobulin-like receptors normally apply, so NK cells attack.
Q 19Natural killer cells make up roughly what share of circulating lymphocytes in humans?
5-20%
NK cells make up roughly 5-20% of circulating lymphocytes in humans and are identified as CD56-positive and CD3-negative.
Q 20What are the four classic symptoms of inflammation?
Redness, swelling, heat and pain
Redness, swelling, heat and pain, Celsus's rubor, tumor, calor and dolor, come from increased blood flow and leaky vessels driven by eicosanoids and cytokines from injured cells.
More than 20 plasma proteins make up the cascade, named for its ability to complement the killing of pathogens by antibodies; it ends in a membrane attack complex.
Q 24Coating a pathogen's surface with complement or antibody to mark it for destruction is called what?
Opsonisation
Opsonisation coats the pathogen with C3b or antibody so engulfing cells bind and swallow it more readily; complement fragments also attract immune cells and increase vascular permeability.
Q 25In 1891 Hans Buchner named the bactericidal property of blood "alexin", from a Greek word meaning what?
To ward off
Alexin comes from the Greek alexein, 'to ward off'; Jules Bordet then split it into a heat-stable sensitiser and a heat-labile part that Ehrlich renamed complement in 1899.
Q 26Toll-like receptors, which detect pathogen-associated molecular patterns, were first discovered in which organism?
Drosophila fruit flies
The Toll gene was found in Drosophila fruit flies, where Jules Hoffmann's lab showed in 1996 that it drives antifungal defence; ten toll-like receptors have since been described in humans.
Q 27Dendritic cells are named for their resemblance to what?
The branches of neurons
Dendritic cells are named for their resemblance to the branches of neurons, the dendrites; found in skin, nose, lungs and gut, they present antigens to T cells, linking innate and adaptive immunity.
Q 28Which granulocyte, resident in connective tissue and mucous membranes, is most associated with allergy and anaphylaxis?
Mast cell
Mast cells sit in connective tissue and mucous membranes and release histamine when IgE on their surface is cross-linked, driving allergy and anaphylaxis; basophils and eosinophils mainly target parasites.
Q 29Which organ acts as a blood filter, removes old red blood cells and holds a reserve of blood?
The spleen
The spleen filters blood, removes worn-out red cells, recycles their iron and holds a reserve of blood; structurally it is built like a very large lymph node.
Q 30Kupffer cells in the liver and microglia in the brain are tissue-resident forms of which cell?
Macrophage
Kupffer cells and microglia are tissue-resident macrophages, a cell type Metchnikoff named in 1884; they all belong to the mononuclear phagocyte system.