<span>Certain enzymes and other chemicals including antibodies are created by immune cells. The white blood cells surround the bacteria or other infectious material and release the chemicals and enzymes into the bloodstream.</span>
The removal of insects would mess up the food chain. Many animals rely on insects as a source of food and energy. Eventually everything would die off.
Cell walls are thicker than cell membranes, and can therefore support the cell well. They keep a strong boundary for the protection of the cell, but can let water and some nutrients through to nourish and power the cell. Hope this helps! :)