system
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human
Can T-lymphocytes of a person be cloned in vitro?

If so, how is it done?

Via bone marrow extraction perhaps?
gpt
T cells can be isolated and cultured directly from peripheral blood. 

Ficoll gradient centrifugation is used to separate peripheral blood mononuclear cells (PBMCs) from other blood components. 

From there, cells of interest (i.e. t cells) can be isolated using antibodies against specific surface markers (e.g. CD3 for t cells) and magnetic beads (see any cell isolation kits from thermofisher/miltenyi for example protocols). 

Bone marrow aspirate could also be used but peripheral blood is more convenient, depends what the purpose of the experiment is.