Assuming you're asking about the human body, they interact in many ways. They can communicate with hormones, such as testosterone. Different parts of the body produce different things for use in different parts, such as red blood cells which come from one's bone marrow. There's also direct mechanical interaction as between bones and muscles pulling on bones. The brain connects to pretty much every part of the body via nerves. The lungs connect to the entire body indirectly through the oxygen they supply and the carbon dioxide they remove. There are many more examples possible. Please be more specific for more.