Revisiting T cells: Innate actions and emerging links to innate memory response

The traditional view of the immune system distinguishes between the innate immune system, which serves as the host's first line of defense against pathogens, and the adaptive immune system, which evolved to manage more complex or recurrent infections. However, the discovery of evolutionarily conserved mechanisms in innate memory cells (trained immunity) has revealed adaptive-like characteristics in innate immune cells, challenging the conventional dogma between innate and adaptive immunity. Simultaneously, growing evidence shows that T cells exhibit innate-like features. In this review, we explore unconventional T cells (γδ cells, natural killer T cells, mucosal-associated invariant T cells) and conventional αβ T cells (virtual memory, memory T cells) to emphasize the broad spectrum of beneficial effects that the adaptive immune system can particularly exert in early host defense strategies, including innate immune memory. These mechanisms confer clear advantages for immune protection and homeostasis, especially during early childhood, and provide a new perspective on canonical immune functions.