How to prevent weakened immunity while eating healthy foods?

Recently, for various reasons, I have been following a personalized diet and trying to eat a lot of healthy foods that suit me (chicken breast, broccoli), but I am worried about a weakened immune system. No matter how good the food is, I hear that you need to eat a balanced diet, so I am concerned that nutrient deficiencies might weaken my immunity. What are some ways to maintain and strengthen immunity during a personalized diet?