Been reading about cytokines in the course of my informal readings on cellular biology. Here is a partially paraphrased description from Wikipedia's entry on this topic:
Cytokines are a category of signalling proteins and glycoproteins that play a vital role in cellular communication. In that respect, they are similar to hormones and neurotransmitters; however, hormones are secreted from specific organs to the blood and neurotransmitters are related to neural activity. Cytokines, in contrast, are a more diverse class of compounds in terms of origin and purpose. They are produced by a wide variety of hematopoietic and non-hematopoietic cell types and can have effects on both nearby cells or throughout the organism, sometimes strongly dependent on the presence of other chemicals.
Cytokines are critical to the functioning of both innate and adaptive immune responses. They are often secreted by immune cells which have encountered a pathogen as a way to activate and recruit more immune cells and increase the system's response to the pathogen. However, apart from their role in the development and functioning of the immune system, as well as their aberrant modes of secretion in a variety of immunological, inflammatory and infectious diseases, cytokines are also involved in several developmental processes during embryogenesis.