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Quiz: The Immune System as a Sense Organ

Test your understanding of Chapter 4. Choose the best answer, then click Show Answer to check.

Question 1

Which statement best describes innate immunity?

  1. A slow, highly specific response that remembers each invader it has met
  2. A fast, general response that recognizes broad patterns shared by many microbes
  3. A response made only by antibody-producing plasma cells in the gut
  4. A response that begins only after the brain sends a signal down the vagus nerve
Show Answer

The correct answer is B. Innate immunity responds within minutes to hours to broad patterns, such as pieces of bacterial walls, and has no memory. Option A describes adaptive immunity, which is slower the first time but specific and able to remember.

Concept Tested: Innate Immunity Bloom's Level: Remember

Question 2

According to the 2023 count by Sender and colleagues, what share of the body's immune cells are in the gut?

  1. About 70% of all immune cells
  2. About 50% of all immune cells
  3. About 20% of all immune cells
  4. About 3% of all immune cells, though about 70% of antibody-making plasma cells
Show Answer

The correct answer is D. The careful 2023 count found about 3% of all immune cells (and about 5% of lymphocytes) in the gut. The popular "70%" figure matches the gut's share of antibody-making plasma cells, not of the whole immune system, so option A mixes up the two.

Concept Tested: Gut-Associated Lymphoid Tissue Bloom's Level: Remember

Question 3

What is a circumventricular organ?

  1. A small brain area near the ventricles that lacks a complete blood-brain barrier
  2. A cluster of immune cells in the small intestine that samples food
  3. A nerve that carries signals from the gut to the brainstem
  4. A layer of mucus that keeps bacteria away from the gut lining
Show Answer

The correct answer is A. Circumventricular organs sit around the brain's fluid-filled ventricles and lack a full blood-brain barrier, so blood-borne signals can reach brain tissue there. Option B describes a Peyer's patch, part of gut-associated lymphoid tissue, which is a different kind of sampling station.

Concept Tested: Circumventricular Organ Bloom's Level: Remember

Question 4

Why does the chapter describe sickness behavior as an evolved strategy rather than simple weakness?

  1. Because sick people always feel better if they keep exercising
  2. Because sickness behavior is caused by bacteria controlling the brain directly
  3. Because resting, eating less, and withdrawing are organized behaviors that help fever fight infection
  4. Because sickness behavior only happens in animals, never in humans
Show Answer

The correct answer is C. Benjamin Hart proposed in 1988 that sickness behavior is an organized, evolved response that helps fever fight viral and bacterial infections, not simply a breakdown. Option D is wrong because a 2010 randomized experiment showed that inflammation lowers mood in healthy humans too.

Concept Tested: Sickness Behavior Bloom's Level: Understand

Question 5

How do tight junctions contribute to the gut's barrier?

  1. They make antibodies that coat bacteria in the gut
  2. They seal the gaps between neighboring lining cells and control what slips between them
  3. They form a thick wall of muscle that pushes bacteria out of the gut
  4. They completely block water and every molecule from passing between cells
Show Answer

The correct answer is B. Tight junctions are protein seals that link neighboring epithelial cells, like mortar between stones. Option D overstates it: the seal is adjustable and lets water and some small molecules through, which is why "leaky gut" claims need careful checking in Chapter 6.

Concept Tested: Tight Junction Bloom's Level: Understand

Question 6

Which description best explains how gut cytokines can reach the brain even though the brain has a blood-brain barrier?

  1. Cytokines dissolve the blood-brain barrier completely during every infection.
  2. Cytokines cannot affect the brain; sickness feelings come only from the gut.
  3. Only bacteria, not cytokines, can send signals to the brain.
  4. Cytokines activate vagus nerve fibers, act at circumventricular organs, are carried across in some cases, and trigger barrier cells to relay signals.
Show Answer

The correct answer is D. Quan and Banks described four routes: the nerve route (especially the vagus), circumventricular organs, transport of some cytokines across the barrier, and signals relayed by barrier cells. Option B reflects the misconception that the brain is sealed off from the immune system.

Concept Tested: Immune-to-Brain Signaling Bloom's Level: Understand

Question 7

A 16-year-old has a stomach virus with fever. She sleeps most of the day, has no appetite, and doesn't want to talk to friends. Her brother says she is just being lazy. Which response best applies this chapter?

  1. Her immune cells are releasing cytokines that signal her brain, producing sickness behavior that should fade as the infection clears.
  2. Her symptoms prove she has depression, which inflammation always causes.
  3. Her gut bacteria are making her choose to stay in bed.
  4. Her symptoms mean her immune system is too weak and needs boosting.
Show Answer

The correct answer is A. Infection triggers inflammation, and cytokines such as IL-1β, IL-6, and TNF act on the brain to cause tiredness, low appetite, and withdrawal. This is temporary and expected. Option B overstates the evidence: inflammation can contribute to depression in some people, but short sickness behavior is not a depression diagnosis.

Concept Tested: Sickness Behavior Bloom's Level: Apply

Question 8

In the Cytokine Feedback Loop MicroSim, a student sets gut inflammation to 0 but sets sleep to 5 hours and stress to 8. The loop is still active. Which change would most likely weaken it most?

  1. Treating gut inflammation, because that always helps most
  2. Raising stress further, to push the loop past its limit
  3. Protecting sleep or managing stress, because those feedback arrows are now driving the loop
  4. Nothing, because loops can never be broken once they start
Show Answer

The correct answer is C. With gut inflammation already at 0, there's little left to treat at the gut; the signal is coming from sleep problems and stress feeding back into cytokines (and, for stress, into gut inflammation). Breaking those links weakens the loop. Option A is a common assumption, but which link matters most depends on where the signal is coming from.

Concept Tested: Cytokine Bloom's Level: Apply

Question 9

In the 2013 infliximab trial for depression, infliximab did not beat placebo overall, but people with high CRP seemed to respond better. What is the most reasonable conclusion?

  1. Anti-TNF drugs cure depression and should be used for everyone with depression.
  2. Inflammation plays no role in depression at all.
  3. The trial proves high CRP causes depression.
  4. Blocking TNF may help a subset of people with high inflammation, but a larger trial designed for that group is needed.
Show Answer

The correct answer is D. The main result was no overall benefit, and the high-CRP finding came from a small, exploratory subgroup (22 people) that could reflect chance. Option B goes too far the other way: other human evidence, such as experimental endotoxin and interferon studies, supports a contributing role for inflammation in some people.

Concept Tested: Tumor Necrosis Factor Bloom's Level: Analyze

Question 10

Germ-free mice have immature microglia, and microbial short-chain fatty acids help restore them (Erny et al., 2015). Why does this chapter grade "Your gut bacteria control your brain's immune cells" as Preliminary?

  1. The results come from mice raised with no microbes at all, and human evidence is missing.
  2. The study was done in thousands of people but had no control group.
  3. The study showed that microglia are made in the bone marrow, not the brain.
  4. Microglia are not immune cells, so the study was measuring the wrong thing.
Show Answer

The correct answer is A. Germ-free mice are an extreme situation no person lives in, and no human study has shown that normal microbe differences change microglia in a meaningful way. Option C is wrong: fate-mapping studies in mice showed that microglia come from early embryonic precursors and maintain themselves in the brain.

Concept Tested: Microglia Bloom's Level: Analyze