How diet autoimmune diseases: how does one affect the others?

Avoid toxic foods. Avoid sugars, fried foods, vegatable oils like canola & soy, meats that are cured wth nitrates & nitrites, aspartame, splenda, partially hydrogenated vegetable oils, maragarine, soy based products, sweetened beverages like juice & soda, microwaved foods, artificial colors & flavors. All of these products cause damage to the cells, tissues & organs in your body contributing to various diseases.
No one factor. There is no good double blind placebo controlled peer reviewed data that any specific foods affects the immune system. A reasonable diet, rest, exercise, less stress, appropriate immunizations, infectious exposures, disease, medications, and genetic luck are contributors.