What is the small cell signaling protein/peptide/glycoprotein secreted by immune cells, used for intercellular communication? Normally present in small amounts; concentrations may increase up to 1,000-fold during trauma or infection. Action is short-lived and exerted through binding to their receptor on the target cell/organ. Activity may be autocrine, paracrine or endocrine. They share pleiotropism, redundancy, synergy and antagnism with other as part of tight regulation of the immune system.