Your immune system is your body's defense network that helps protect you from getting sick. It's made up of special cells, tissues, and organs that work together like an invisible army, constantly patrolling your body to find and destroy germs, viruses, and other invaders that could make you ill.

When a virus enters your body through your nose or a cut on your finger, your immune system springs into action. Special white blood cells rush to the infection site like soldiers responding to an alarm. Some cells identify the invader, others attack it directly, and still others remember it so your body can fight it off faster if it ever returns. This is why you usually only get chickenpox once: your immune system remembers that virus and destroys it quickly if it tries to invade again.

Your immune system learns and adapts throughout your life. Vaccines work by training your immune system to recognize dangerous diseases without actually making you sick, like a fire drill prepares you for a real emergency. When you recover from a cold, your immune system has learned to fight that particular cold virus.

Sometimes an immune system can be overactive, attacking harmless things like pollen or peanuts, which can cause allergies. Other times it might be weakened by illness or medicine, making someone more vulnerable to infections. A healthy immune system can stay strong with good nutrition, enough sleep, and exercise.