1. Discuss the process that occurs in signal transduction cascades. In the animation illustrating T-cell proliferation, signal transduction begins when a foreign antigen carried by the antigen-presenting cell (APC) is recognized by the T-cell receptor. Several molecules located in or near the cell membrane mediate the transmission of signals, resulting in the production of inositol triphosphate (IP 3 ), which travels into the cell's cytoplasm. IP 3 initiates the release of calcium, which, through several intermediate steps, allows a transcription factor to enter the nucleus. Within the nucleus, this transcription factor and other essential factors trigger the expression of the gene interleukin-2 (IL-2). IL-2 protein is produced and released extracellularly, where IL-2 binds to receptors on the T cell. This receptor/IL-2 protein complex signals the T cell to proliferate.