This page has a nice graphic that explains how sugar reduces the efficiency of your immune system.
When reading any piece of information, like a web page or a study, it's important to note the difference between correlation and causation. Years ago there was a big flap about aluminum possibly causing Alzheimer's disease. What the study said: "Aluminum found in x% of dead Alzheimer patient brains." What the journalist wrote: "ZOMG! Aluminum causes Alzheimer's!!!"
Americans panicked, people looked for deodorants without aluminum, and replaced their aluminum cooking pots and pans with something else. These are the people who do not understand correlation vs causation. And the deodorant and pan companies made money off of them.
First of all, if someone uses "ZOMG" they are probably under the age of 16. Second, if a person uses more than one exclamation point in a paragraph, it's safe to assume they are over reactive, or trying to sell you something you don't need. In either of these cases you cannot trust the source.
There was some correlation between aluminum in dead brains and Alzheimer patients, but causation is much harder to prove, and causation was not proven in this case. Another possible correlation was 100% of the dead Alzheimer patients brains contained dihydrogen monoxide, a deadly chemical known to almost everyone. But this is correlation, not causation. No one showed that water killed these people. There was no cause and effect proven.
Here's another example. If you hold your hand over a lit candle flame, you will eventually get burned. Now that's a working theory at this point, but I think science has proven pretty well many times that excessive heat causes burns on human tissue. So it's a pretty good bet we have proven causation.
"It takes only about 2 tablespoons of sugar to weaken the immune system by up to 92%."
Now look closely at that statement, which says "up to 92%". That's the worst case scenario, not the average case. Still, sugar is pretty bad. Most people eat too much, that's a big reason it's bad.