The cascade of processes by which a signal interacts with a receptor, causing a change in the level or activity of a second messenger or other downstream target, and ultimately leading to the activation, perpetuation, or inhibition of an immune response. [ http://www.ncbi.nlm.nih.gov/pubmed/15771571 https://www.worldcat.org/search?q=bn%3A0781735149 GOC:add ]