The mechanisms behind the extreme sensitivity and specificity of such broadly reactive receptors are intriguing and will likely be important to understand antigen receptor function in immune responses and in abnormal Dorsomorphin processes such as autoimmunity or

lymphocyte cancers. In their architecture, antigen receptors are multichain complexes. They contain the clonotypic antigen-binding chains (TCR-α and TCR-β chains or BCR immunoglobulin (Ig) heavy and light chains) and constant signalling chains (two CD3 dimers and one TCR-ζ dimer for the TCR, the Ig-αβ heterodimer for the BCR).1,2 The first detectable biochemical step of antigen receptor activation is tyrosine phosphorylation of the cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs) by Src family kinases. The initial phosphorylation leads to recruitment of Syk/ZAP70 kinases, their substrates and signalling enzymes that eventually bring about lymphocyte activation. The exact mechanisms by which antigen binding

triggers these biochemical steps are highly debated and have been the subject of a number of excellent reviews.3–7 In vivo, lymphocytes continuously scan tissues for the presence of antigen displayed on antigen-presenting cells (APCs). Landmark imaging of T cells interacting with APCs revealed that T cells form a specialized contact with the APCs, called the immunological synapse.8,9 The synapse is characterized by accumulation of the TCR in the centre, EX 527 cell line with a surrounding ring of adhesion molecules. This pattern of receptor organization

was later extended to B cells10 and cytotoxic T cells11 and suggested that spatial organization in the immunological synapse may provide Paclitaxel chemical structure a common layer of fidelity for lymphocyte activation.12,13 Imaging of the formation of the immunological synapse showed that the accumulation of antigen receptors in the centre of the synapse is preceded by microclustering of the antigen receptors in the periphery (Fig. 1).14–16 Once formed, the microclusters are transported to the centre of the synapse by an actin-dependent process. The synaptic microclusters appear to be the platforms for receptor activation and signal propagation. For example, microclusters recruit signalling molecules such as Src kinases and ZAP-70/Syk. They also exclude inhibitory phosphatases such as CD45. However, many of the molecular mechanisms of antigen receptor activation inside these structures remain beyond the resolution of optical microscopy and could not be directly addressed by conventional imaging.7,17 Recently, several techniques based on fluorescence microscopy offer imaging with resolution that approaches the molecular scale (5–40 nm).18–20 The most accessible of these new techniques have been photoactivated localization microscopy (PALM)21 and the related stochastic optical reconstruction microscopy (STORM),22 which are based on the detection and precise localization of single molecules.