What is the Immune System?

The immune system is the body's defense network against pathogens like bacteria, viruses, and parasites. It has two main layers: innate immunity, a fast and general first response, and adaptive immunity, a slower but highly specific response that creates lasting memory. Together they protect individuals — and, through herd immunity, entire populations.

Short answer

The immune system defends the body using innate defenses (skin, inflammation, phagocytes) for immediate, general protection and adaptive defenses (T cells, B cells, antibodies) for targeted, long-lasting immunity.

Innate vs Adaptive Immunity

Innate Immunity

•Present from birth
•Fast, non-specific response
•Skin, mucous membranes, phagocytes
•No memory

Adaptive Immunity

•Develops after exposure
•Slower but highly specific response
•T cells and B cells, antibodies
•Creates immunological memory

Try it: interactive calculator

Herd immunity threshold

93.3%

= (1-1/15)*100

Basic reproduction number (R₀)15

Step-by-step worked examples

Measles has a basic reproduction number (R₀) of about 15. What percentage of the population needs immunity to reach herd immunity?

HIT = 1 − 1/R₀
= 1 − 1/15
= 1 − 0.067 = 0.933
HIT ≈ 93.3% of the population needs immunity

Seasonal influenza has a lower R₀ of about 1.3. Find its herd immunity threshold.

HIT = 1 − 1/1.3
= 1 − 0.769
= 0.231
HIT ≈ 23.1% — much lower than measles because flu spreads less easily

Mumps has an R₀ around 5. How does its herd immunity threshold compare to measles (93.3%)?

HIT = 1 − 1/5 = 1 − 0.2 = 0.8 → 80%
Compared to measles: 93.3% − 80% = 13.3 percentage points lower
A lower R₀ means fewer people need to be immune to stop spread

Flashcards

Quick quiz

Q1.Which immune response acts within hours and lacks memory?

Correct answer: B. Innate immunity is the fast, non-specific first line of defense with no memory of prior exposures.

Q2.Measles has an R₀ of about 15. What's its herd immunity threshold?

Correct answer: C. HIT = 1 − 1/R₀ = 1 − 1/15 ≈ 93.3%.

Q3.Which cells produce antibodies?

Correct answer: C. B cells differentiate into plasma cells that produce antibodies specific to a pathogen.

Q4.Why is the secondary immune response faster than the primary one?

Correct answer: B. Memory cells created after the first exposure allow a much faster, stronger response the second time.

📄Download this topic as a printable worksheet (PDF)Summary + 10 questions + answer key — print it, share it in class.↓

Common mistakes

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The immune system only has one type of defense. — Correct: It has two: fast, general innate immunity and slower, specific adaptive immunity that work together.

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A higher R₀ means a lower herd immunity threshold. — Correct: It's the opposite — a higher R₀ (more contagious) requires a HIGHER immunity threshold: HIT = 1 − 1/R₀.

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Antibodies are produced immediately on first exposure. — Correct: The primary antibody response takes about 10-14 days to peak; only the secondary response is fast.

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Vaccines work completely differently from natural infection. — Correct: Vaccines create the same kind of memory B and T cells as infection, just without the disease risk.

FAQ

What is the immune system?

It's the body's defense network — innate and adaptive immunity working together to detect and destroy pathogens.

What is the formula for herd immunity threshold?

HIT = 1 − 1/R₀, where R₀ is how many people one infected person typically infects.

What are examples of immune system function?

A fever fighting infection (innate), antibody production after a vaccine (adaptive), and herd immunity from mass vaccination are examples.

How do you calculate the herd immunity threshold?

Subtract 1 divided by the pathogen's R₀ from 1, then multiply by 100 for a percentage — try it in the calculator above.