Believe it or not, there was a time before iPhones. But people still needed to communicate with each other. What did people do in this pre-texting era to get the message across? They might have used a pair of tin cans and a piece of string. They also might have used a wall-phone with a rotary dial. Both were handy ways to communicate about who was and wasn't attending the sock hop.
Our bodies don't have tin cans connected by string for conversation. (Thankfully, we are slightly more advanced.) However, our body does communicate between its various systems. While all of these systems use a wide range of chemicals and processes to spread messages across the body, just remember that we are still talking about the same fundamental idea of two tin cans and a string.
The immune system's communication system all begins with some sort of outside stimulus, usually an unwelcome one in the form of an invading alien pathogen. Immune cells sense these odd-looking fellas and release different kinds of chemicals, usually called cytokines, to trigger some sort of defensive response. These chemicals signal to other immune cells to get their acts together and help out.
The immune system is also the thing that gives you a fever when you are sick. While a skyrocketing temperature may prevent you from going outside to frolic, your hot forehead is the sign of a body hard at work fending off invaders.
When we're talking about the endocrine system, hormones are the stuff of communication. These chemicals get released from an endocrine gland and act at a different part of the body. This communication is pretty darn important too, since hormones are involved in just about every bodily process from metabolism to growing big and strong.
The nervous system does a lot of communicating, too. To convey an idea or evoke a particular response, chemical neurotransmitters do all the talking. You know how you get a bit sweaty when you see that special someone walking in front of you in the hallway? That happened all because your brain communicated to your periphery, to let your skin know that it had noticed the cutie up ahead.