Immune response activation. Illustration of antigen-presenting cell activity. Dendritic cells (bright blue) are shown presenting antigens to a lymphocyte (centre) through their membrane-bound major histocompatibility complex (MH) molecules (shown in inset at upper left). Also shown are CD4 molecules (light blue) binding to other portions of the MHC, strengthening the interaction. After binding to the MHC-antigen complex, The T-cell receptor (blue) sends a signal cascade through an attached G-protein into the T-lymphocyte cell, that activates an immune response. This is a key part of the molecular basis of the human immune system's response to disease and infection.