immune system

• Cell signalling 


  basically how cells communicate. A cell may release (or present) a substance that binds to the receptors on another cell - this causes a response of some kind in the other cell.
• Cell signalling is really important in the immune response because it

  helps to activate all the different types of white blood cells that are needed.
• T helper cells release interleukins (a type of cytokine) that bind to receptors on B lymphocytes. This activates the B lymphocytes - the T helper cells are signalling to the B lymphocytes that there's a pathogen in the body.