In the immunological synapse formed between an antigen-presenting cell (APC) and a T cell, the activation of antigen-specific T cells requires three signals.

Signal 1: Consists of antigenic peptides presented by MHC class II molecules. This signal is recognized by the antigen-specific T cell receptor (TCR).

Signal 2: Stabilizes the immunological synapse through adhesion molecules and is generated by interactions between co-stimulatory molecules on the APC and the T cell surface. Binding of CD80/CD86 on the APC to its receptor CD28 on the T cell delivers an activating signal, while interaction with cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) generates an inhibitory signal.

Signal 3: Formed by cytokines secreted by the APC. These cytokines transmit signals through cytokine receptors on the T cell surface, promoting the differentiation of T cells into effector phenotypes.