No magic pill. Many people are looking for some "magic pill" to "boost their immune system". There is no such pill. However, there is a way to boost your immune system, live a healthy lifestyle. Proper diet, proper sleep- wake cycles, healthy environment, proper exercise are the foundation of good health and good immune health. Supplements should be used selectively to supplement a healthy diet.
Not proven effective. Many supplements or foods claim they can boost one's immune system, but none are proven to actually do this in humans. A pregnant woman does her best by eating a healthy, balanced diet with plenty of vegetables, taking her prenatal vitamins, staying happy and relaxed, and avoiding contact with ill persons. In pregnancy, the immune system is adjusted, so not to reject the fetus living inside "mom".