A model of naive T-cell trafficking and surveillance. A naive T cell enters one of N LNs at random, from the blood. It transits the node making contacts of duration τ with DCs (shown in blue), surveying for antigen; it egresses with a constant probability per DC encounter, returning to blood after a mean transit time T^L. (If q is the probability of egress per DC encounter, T^L = τ/q.) It then spends an average time T^B in the blood before re-entering a LN, again at random. In 1 node (orange), antigen is present on a proportion f of DCs (purple) and contact between a T cell and antigen-bearing DCs results in detection. The rate of egress from this node may also be reduced in an infection scenario.