There are some things that are bad your immune system:
Not enough exercise
If you sit at a desk doing nothing all day feeling drowsy, you immune system will be drowsy also. Even small amounts of exercise will improve the immune system and increase levels of leukocytes.
Excess fat cells in floating around in the body will affect your immune system. Pro-inflammatory chemicals are released by high numbers of fat cells. When the inflammation happens, the healthy tissues in the body become damaged.
Consuming too much sugar or fat
Too much sugar lowers the number of immune system cells which would normally attack bacteria in the body. Even about 2 cans of fizzy drink will reduce the ability of white blood cells.