Too Much Sugar: Eating or drinking 100 grams (8 tbsp.) of sugar, the equivalent of about two cans of soda, can reduce the ability of white blood cells to kill germs by forty percent. The immune-suppressing effect of sugar starts less than thirty minutes after ingestion and may last for five hours.
Excess Alcohol: Alcohol can harm the body's immune system by producing an overall nutritional deficiency, depriving the body of valuable immune-boosting nutrients. In addition, alcohol consumed in excess can reduce the ability of white blood cells to kill germs. One drink does not appear to bother the immune system, but three or more drinks do.
Food Allergens: Sometimes the immune system recognizes an otherwise harmless substance (such as milk) as a foreign invader and attack it, causing an allergic reaction. After many encounters with food allergens, the damage enables invaders and other potentially toxic substances to get into the bloodstream and make the body feel miserable. This condition is known as the leaky gut syndrome.
Too Much Body Fat: Obesity can lead to a depressed immune system. It can affect the ability of white blood cells to multiply, produce antibodies and rush to the site of an infection.