<unk> s can respond to novel pathogens that the immune system has not yet encountered. Recognition by a<unk>  clone of its cognate antigen results in the initiation of an immune response. In turn, this results in the T<unk>  acquiring an activated phenotype seen by the up-regulation of surface markers CD25+, CD44+, CD62Llow, CD69+ and may further differentiate into a memory T<unk> . Having adequate numbers of<unk> s is essential for the immune system to continuously respond to unfamiliar pathogens.
Naive T cell
Nonspecific immune cell
Reactive lymphocyte
Memory B cell
Regulatory B cell
Regulatory T cell
Chimeric antigen receptor T cell
Natural killer T cell