Tea
Black tea contains L-theanine, an immune-boosting amino acid. Both black and green tea also contain catechins, antioxidants that have been found to possibly prevent the flu.
Beef
Zinc is essential for the development of white blood cells, the intrepid immune system cells that recognize and destroy invading bacteria, viruses, and assorted other bad guys, says William Boisvert, Ph.D., an expert in nutrition and immunity at The Scripps Research Institute in La Jolla, CA. Beef is a good source of zinc, as are milk and beans.