Before exposure to a foreign antigen, T cells with receptors specific for the antigen are a tiny fraction of the T cells—on the order of 1 in 10⁵ or 1 in 10⁶ T cells. After exposure to the antigen, only a small number of dendritic cells typically display the antigen on their surface. How long does it take for such antigen-presenting dendritic cells to interact with the antigen-specific T cells, which is the key first step in T cell activation and clonal expansion? The dynamics of the search process were examined by labeling dendritic cells red and T cells green, so that contacts in an intact lymph node could be scored visually using two-photon fluorescence microscopy (Figure Q24–3A). The frequency of contacts between dendritic cells and T cells from such experiments is given in Figure 24–3B. Assuming that 100 dendritic cells present the specific antigen, how long would it take them to scan 10⁵ T cells? How long for 10⁶ T cells?

Figure Q24-3

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(A) 1 3 2 2 3 0:35 1:20 2:30 (B) 600 400 200 10 20 30 40 50 60 minutes encounters