Wow, this is big!
Drinking a Coke or eating a candy bar can partially knock out your immune system for up to 5 hours.
How come nobody has ever told me this before?
"...in adults, cell mediated immunity is significantly depressed after sugar ingestion (75 grams). A 100g portion of sugar can significantly reduce the capacity of white blood cells to engulf bacteria. Maximum immune suppression occurs one to two hours after ingestion and remains suppressed for up to five hours after feeding."
Source
Explanation why sugar lowers the effectiveness of the immune system:
"It was only in the 1970's that researchers found out that vitamin C was needed by white blood cells so that they could phagocytize viruses and bacteria. White blood cells require a 50 times higher concentration inside the cell as outside so they have to accumulate vitamin C.
There is something called a "phagocytic index" which tells you how rapidly a particular macrophage or lymphocyte can gobble up a virus, bacteria, or cancer cell. It was in the 1970's that Linus Pauling realized that white blood cells need a high dose of vitamin C and that is when he came up with his theory that you need high doses of vitamin C to combat the common cold.
We know that glucose and vitamin C have similar chemical structures, so what happens when the sugar levels go up? They compete for one another upon entering the cells. And the thing that mediates the entry of glucose into the cells is the same thing that mediates the entry of vitamin C into the cells. If there is more glucose around, there is going to be less vitamin C allowed into the cell. It doesn't take much: a blood sugar value of 120 reduces the phagocytic index by 75%. So when you eat sugar, think of your immune system slowing down to a crawl."
Source