Before exposure to a foreign antigen, T cells with receptors specific for the antigen

Chapter 0, Problem 24-9

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Before exposure to a foreign antigen, T cells with receptors specific for the antigen are a tiny fraction of the T cellson 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 Q243A). The frequency of contacts between dendritic cells and T cells from such experiments is given in Figure 243B. 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?