Advances in understanding of complex biological systems crucially depend on the development of biophysical methods, such as super-resolution fluorescence microscopies and spectroscopies, which provide detailed information on structure and dynamics at the molecular level. We develop such advanced optical techniques and use cell-mimicking model systems to better understand the processes within living cells. In the seminar, I will present our recent investigations of molecular organisation within biological membranes, in particular the biophysical interplay between lipids and proteins involved in the activation of T lymphocytes – the key step of our immune response.