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F. Baumgart, M. Schneider, G. Schütz:
"How T Cells Do the "Search for the Needle in the Haystack"";
Frontiers in Physics, 7 (2019), 11; 1 - 8.

In the body, a T cell is confronted with millions of antigen-presenting cells (APCs) in the
search for potentially harmful antigen. To elicit an appropriate immune response, this
search has to be performed as fast and as precise as possible. These two requirements,
however, are at odds with each other: fast searches lack accuracy, whereas high fidelity
decisions are typically time-consuming. Here, we use the archetypical search for the
needle in the haystack as an analogy for the T cell´s search problem. We provide a
statistical framework to quantitatively estimate the constraints of search strategies for rare
instances. Particularly, we propose a solution for balancing the demand for high speed
with low error rates. It takes advantage of a two-phase search process, which combines
a first rapid scan with a second high-fidelity check. Finally, we provide arguments that
support a two-phase search model for identification of antigen-positive APCs by T cells.

http://dx.doi.org/10.3389/fphy.2019.00011