We discuss continuum (lattice-free) and inherently stochastic models of immune cellular interactions, using the simplest hypothesis, that cells follow Brownian paths. The timescale for a cell to explore a volume such as a lymph node is L 2 =D, where L is the radius of the region and D the diffusivity of a cell. The average time a cell spends in a volume with an exit is proportional to L 3 =aD, where a is the radius of the exit. The mean time before a cell encounters a zone of attraction with radius b around, for example, an antigen presenting cell, is proportional to L 3 =bD.