Immune system (noun): the body’s natural defence network that protects you from illness, infection, and harmful substances by detecting and fighting threats.

At its simplest, the immune system is your body’s security team. It constantly checks what belongs in your body and what doesn’t. When it detects danger — viruses, bacteria, or toxins — it reacts, attacks, and remembers.

What makes the immune system special is that it learns. Once it has fought an illness, it often remembers it and responds faster next time. This is why vaccinations work and why some diseases don’t affect you twice in the same way.

In everyday language, people also talk about the immune system more loosely — as something that can be strong, weak, boosted, or compromised, often connected to stress, sleep, diet, and lifestyle.

MEANING 1: The Body’s Defence Against Disease — VERY COMMON

This is the core meaning. The immune system identifies and destroys harmful invaders. When it works well, you recover quickly or don’t get sick at all. When it’s weak, infections last longer or happen more often.

MEANING 2: General Physical Resistance or Health (Everyday Use)

In everyday speech, people use immune system to mean overall resistance and vitality. Saying “my immune system is low” usually means “I get sick easily” or “my body feels run down,” not a medical diagnosis.

Examples from the street:

• “My immune system is weak right now” → I’m getting sick easily
• “Sleep really helps your immune system” → rest improves resistance
• “The immune system fights infections” → it protects the body

2. Most Common Patterns

Immune system as health defence — VERY COMMON:

• the immune system fights/responds to + infection → defends the body
• a strong/weak immune system → level of resistance
• support/boost the immune system → improve defence

Immune system under pressure:

• the immune system is compromised → defence is weakened
• stress affects the immune system → mental pressure harms resistance

3. Phrasal Verbs

Note: “Immune system” does not form phrasal verbs — these are common related expressions:

• fight off → successfully resist illness
  Example: “Her body fought off the infection.”
• come down with → become ill
  Example: “He came down with a cold.”
• wear someone down → weaken over time
  Example: “Stress can wear your immune system down.”

4. Example Sentences

1. The immune system fights infections automatically
   → The body defends itself without conscious effort.
2. A strong immune system helps you recover faster
   → Resistance shortens illness.
3. Stress can weaken the immune system over time
   → Mental pressure harms physical defence.
4. Sleep plays a major role in supporting the immune system
   → Rest strengthens resistance.
5. His immune system was compromised after illness
   → His body was more vulnerable.
6. Vaccines train the immune system to respond faster
   → The body learns to defend itself.
7. Poor diet can weaken your immune system
   → Nutrition affects resistance.
8. Children’s immune systems are still developing
   → Their defence systems are not fully mature.
9. Exercise helps keep the immune system balanced
   → Movement supports healthy function.
10. A weakened immune system leads to frequent illness
    → Low resistance causes repeated sickness.

5. Personal Examples

1. In schools, stress and poor sleep can weaken students’ immune systems
   → Academic pressure affects health.
2. While learning English, long-term stress can quietly impact the immune system and energy levels
   → Mental strain affects physical wellbeing.

6. Register: Neutral

✔ Native usage tips

• Immune system is common in both medical and everyday speech
• People often use it loosely to mean general health
• “Boost your immune system” is common but informal
• In serious contexts, doctors avoid vague language and give specifics

✔ Similar expressions / words

• Body’s defences → informal explanation
• Immunity → state of protection
• Resistance → general ability to withstand illness