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.
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.
- •Present from birth
- •Fast, non-specific response
- •Skin, mucous membranes, phagocytes
- •No memory
- •Develops after exposure
- •Slower but highly specific response
- •T cells and B cells, antibodies
- •Creates immunological memory
Try it: interactive calculator
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?
Q2.Measles has an R₀ of about 15. What's its herd immunity threshold?
Q3.Which cells produce antibodies?
Q4.Why is the secondary immune response faster than the primary one?
The full card deck, worked steps and AI-tutor support for “What is the Immune System?” are in Notek — study by hand before your exam.
Common mistakes
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.
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.
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.
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.