Do children aged 4 have immature immune systems ? how can one find out if their immune system is weak ?

Yes. After the age of 2 they have a mature & robust immune system. If they are thriving/playing/growing and not hospitalized on a regular basis they have a normal immune system. Kids place their contaminated fingers in their nose/mouth & near there eyes dozens of times daily. Estimates cite they are challenging their immune system a hundred or more times daily with that behavior.

If your 4 year old child has not had a pneumonia, a sinusitis, a hospitalization or more, is growing, gaining weight and meets developmental milestones, immune system weakness is highly unlikely. Rest easy and be sure to wash hands often.