  Immune cells coordinate their efforts for the correct and efficient
functioning of the immune system (IS). Each cell type plays a distinct role and
communicates with other cell types through mediators such as cytokines,
chemokines and hormones, among others, that are crucial for the functioning of
the IS and its fine tuning. Nevertheless, a quantitative analysis of the
topological properties of an immunological network involving this complex
interchange of mediators among immune cells is still lacking. Here we present a
method for quantifying the relevance of different mediators in the immune
network, which exploits a definition of centrality based on the concept of
efficient communication. The analysis, applied to the human immune system,
indicates that its mediators significantly differ in their network relevance.
We found that cytokines involved in innate immunity and inflammation and some
hormones rank highest in the network, revealing that the most prominent
mediators of the IS are molecules involved in these ancestral types of defence
mechanisms highly integrated with the adaptive immune response, and at the
interplay among the nervous, the endocrine and the immune systems.
