Islands in the Stream: Immunological Information Processing

The human immune system comprises on the order of 1010 leukocytes (“white cells”) of about a dozen distinct types, most of which are mobile and migrate throughout the vasculature and tissues as a kind of “fluid” of independent cells. Each cell bears receptors that bind to conserved host-encoded ligands as well as receptors that recognize microbe-encoded ligands. Lymphocytes, members of an important leukocyte subset, also bear receptors encoded by genes assembled by a stochastic process, and thus have no specific ligands at all. Each leukocyte integrates signals from these various receptors, and alters its behavioral programs accordingly. One component of this response is the induction of its own signaling through diffusible cytokines and surface molecules. The information processing begins with individual cell responses, but comes quickly to involve large numbers of cells responding to each other as well as to environmental signals. These information processing collectives organize themselves into high-density aggregates capable of performing more complex information processing tasks, eventually leading, for example, to the establishment of a cellular memory of the microorganism that induced the immune response. I will present a tutorial on this topic, focusing on the ontogeny and function of one inducible cellular aggregate of particular importance: the germinal center.

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