Final answer:
A mature lymphocyte is called 'naïve' because it has not yet come into contact with an antigen, and thus, is not activated or has any memory of previous infections. It is fully functional and ready for exposure to pathogens for initiating immune responses.
Step-by-step explanation:
When a mature lymphocyte is called 'naïve' it is because it has not yet encountered an antigen to respond to. Naïve lymphocytes are fully functional and are capable of immune responses but require antigen exposure to become activated. Once activated, depending on their type, they may help in the humoral immune response by producing antibodies (B cells) or the cell-mediated immune response (T cells).
Specifically, the correct answer is that a naïve lymphocyte has not come into contact with pathogenic antigens, which means it has neither initiated an immune response nor gained immunological memory from previous infections. Maturation of lymphocytes involves a selection process ensuring they can recognize and bind to a specific antigen, determined by the individual's genetics, hence their readiness for activation upon encountering a matching antigen.