The Immune System
A layered defence that tells "you" from "not-you". How innate and adaptive immunity work together to protect the body…
4.7
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Anatomy Category
Defense & Hormones
Your invisible security team and chemical messengers — the immune system, the endocrine glands and the hormones they release.
Guides
Guides in Defense & Hormones
Start with any guide below — each one is written for beginners and builds on the last.
Hormones Explained
Chemical messengers that travel in the blood to shape mood, growth, sleep and hunger. What hormones are and how they…
4.6
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The Endocrine System
The network of glands that release hormones. Meet the pituitary, thyroid, adrenals and pancreas and see how they…
4.9
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Body Temperature Regulation
Why you sit near 37°C and how sweating and shivering defend that set point. The body's built-in thermostat, explained…
4.9
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Practice
Related tools
Reinforce what you learn with these interactive tools.
Interactive
Body Systems Explorer
Browse all major body systems in one place and jump to the organs and guides for each.
4.6
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Quiz
Anatomy Quiz
Test your knowledge of the human body with an adaptive multiple-choice quiz and instant, explained feedback.
4.9
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Reference
Anatomy Glossary
A searchable, plain-language glossary of the anatomy terms used across the site.
4.8
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Related articles
Deeper dives and fascinating facts from the blog.
How Hormones Work: The Body's Chemical Messages
Hormones travel in the blood to control mood, growth, sleep and hunger. Here is how these tiny messengers run the body.
The Immune System Guide: Your Invisible Defenders
A layered defence protects you every second of the day. Meet the cells and barriers that tell "you" from "not-you".
Body Temperature Explained: Your Built-In Thermostat
Why does the body sit near 37°C, and how do sweating and shivering defend that number? The science of thermoregulation.
Explore another body system
Every part of the body connects to the rest. Browse all nine categories and keep exploring.