Immunological synapse, illustration. Crosstalk between different kinds of immune cells (purple) is important for regulating the immune system. The immunological synapse is a specialized cell-to-cell junction between lymphoid immune cells and an antigen-presenting cell (APC). Initiation of an antigen-specific immune response is based on the interaction between T cell receptors and major histocompatibility complex (MHC) proteins that have bound antigenic peptides (pink).