There are two parts to your immune system, called “non-specific” (this responds right away to toxic exposure, bee stings, trauma, etc) and “specific” (which takes some time, but is more targeted against specific invaders). Think of non-specific immunity like a sledge hammer, while specific immunity is more like a scalpel. The sledge hammer is quick and dirty, and (as you might imagine) will cause a lot of inflammation even in surrounding healthy tissues. The scalpel, on the other hand, won’t cause as much collateral damage, but it will have to be very carefully directed in order to do any good.
Because your specific immune system (the scalpel) needs careful direction, two kinds of cells are necessary: the T and B cells.
If I can mix my metaphors, think of the T cells as the managers and the B cells as the worker bees (no pun intended) – that is, the B cells actually produce antibodies against specific invaders, while the T cells tell the B cells what to do.
Now there are three types of substances (called antigens) that have the potential to provoke your specific immune system to make antibodies against them. There are soluble antigens (these come from your diet and your environment), insoluble antigens (these come from microbes and pathogens), and self antigens (these come from your own cells).
Reactions against soluble antigens are called allergies.
Reactions against self antigens are called autoimmune diseases.
Reactions against insoluble antigens is normal immune function.