While it may seem that orange juice would help, the spike in your blood sugar levels that it causes actually distracts your immune system.
Better to drink tea, perhaps with raw honey, when you want to boost your immune system. Green tea, licorice tea, Throat Coat tea (by Traditional Medicinals – often sold at grocery stores), and even chamomile tea have been shown to support your immune system.
Instead of juice, eat whole fruit (frozen berries, for example).
For tips on fending off a cold or flu, click here for my Natural Medicine Cabinet recommendations.