Molecular pathways of T cell activation

Triggering T cell activation: A quiescent T cell must encounter an antigen presenting cell to become activated. The T receptor for the antigen deliver s a first specific signal which alone is insufficient to trigger total T ce ll activation. A second signal independent of antigen recognition is furnis hed by the antigen presenting cell, leading to total activation. Pathways capable of delivering the second signal: CD28 and CTLA-4 are two m olecules capable of playing a powerful role in regulating immune response. The fate of B cells depends on the CD40 molecule they carry on their surfac e as well as on T cells which express its ligand. Adherence molecules CD11a /CD18 induce tight adherence between T cells and antigen presenting cells b y binding their CD50 ligand (ICAM-3); this bond activates the T cell, compl eting the first signal. The CD2 pathway and its ligands, a constitutional e xpression on two partner cells, is a strong adherence pathway which also tr ansmits two powerful activation signals for T cells. Several functional states: These different activation pathways illustrate t he subtle interaction between surface molecules, capable of inducing differ ent functional stares depending on the environment.