The three signals of T cell activation are:
1. First signal: Initiated by the interaction between antigen-specific T cell receptors on T cells and antigen peptide MHC molecular complexes on antigen-presenting cells, known as the antigen recognition process. This is the initial signal necessary for T cell activation, but this signal alone cannot cause complete activation and proliferation of T cells.
2. Second signal: Provided by the interaction between the co stimulatory molecules on the APC and the corresponding receptor molecules on the surface of T cells, the most important of which is the binding of CD28 molecules on the surface of T cells to the corresponding ligands B7-1 and B7-2 on the APC surface. This signal is crucial for the complete activation and proliferation of T cells. Without this signal, T cells may be in a state of incompetence due to insufficient activation, and even undergo apoptosis.
3. Third signal: Mediated by cytokines secreted by cells, such as IL-2. These cytokines play an important promoting role in T cell activation, which can facilitate the clonal expansion of T cells and induce T cell differentiation into effector cells and memory T cells. The presence of cytokines is a key factor in the further proliferation and differentiation of T cells after activation.