How is the brain involved in the immune system?

A. Immune cells that constitute body's biological defense against infections and toxins, have many things in common with nerve cells. Like nerve cells, immune cells communicate with one another through connections known as synapses. They also have "memory," a molecular switch which enables them to remember an infectious agent and single it out for attack if it tries to invade the body. Additionally, it's now clear that substances known to be important for nerve cell maintenance and survival also support immune system functioning.