Final answer:
Naïve T cells become activated through three signals: first, antigen recognition via the T cell receptor; second, co-stimulation from APCs; and third, cytokines that stimulate proliferation and differentiation.
Step-by-step explanation:
“Naïve” T cells require three signals for their first activation. The first signal involves the recognition of an antigen presented by an antigen-presenting cell (APC) through the Major Histocompatibility Complex (MHC) which binds to the T cell receptor (TCR) on the naïve T cell. Signal 1 is this antigen recognition step.
The second signal, often termed Signal 2, is provided by co-stimulatory molecules that are also expressed on the surface of APCs. These co-stimulatory molecules interact with corresponding receptors on the T cell, serving as a necessary confirmation step that prevents accidental activation.
The third and final activation signal for a T cell is Signal 3, which involves cytokines. These cytokines are signaling proteins released by cells, and in the context of T cell activation, they are often secreted by the APC or other nearby cells. These cytokines modulate T cell responses, further stimulating their proliferation and differentiation into effector cells.
These three signals are critical in ensuring that T cells are activated only under appropriate immune conditions, thus preventing an immune response to self-tissues and maintaining homeostasis within the body’s immune system. For a naïve T cell to transition to an activated T cell ready to fight an infection or cancer, it requires a sequence involving antigen recognition, co-stimulation, and cytokine signals. This forms the basis of an adaptive immune response.