Eating too much simple sugar can negatively affect your immune system for up to 5 hours after you eat it, according to AskDrSears.com. An excess of sugar blocks your immune system from functioning properly and diminishes the ability of white blood cells to fight bacteria. In contrast, complex carbohydrates do not have these effects. Dr. Sears reports that just eight tablespoons of sugar, or 2 1/2, 12-ounce cans of soda, reduces your immune system's ability to fight germs