A composition comprising a) an anti-linker/label epitope chimeric antigen receptor (anti-LLE CAR) comprising I) an extracellular linker/label epitope (LLE) binding domain, II) a transmembrane domain, and III) a signal transduction domain, wherein said extracellular LLE binding domain binds a target cell binding molecule (TCBM), b) said target cell binding molecule (TCBM) comprising i) an antigen binding moiety (ABM), wherein said ABM binds specifically to an antigen expressed on a target cell, ii) a label moiety (LaM), wherein said LaM is a naturally occurring molecule in a subject or a derivative thereof, wherein said naturally occurring molecule is in the circulatory system of said subject, iii) a linker moiety (LiM) conjugating said ABM and said LaM, thereby forming a linker/label epitope (LLE), wherein said LiM is selected from the group consisting of peptides, alkyl chains, alkanoic acids, carboxylic acids, farnesyls, polyethylene glycols, or a derivative thereof, wherein said extracellular LLE binding domain binds said LLE with a higher preference than said naturally occurring molecule, wherein said extracellular LLE binding domain binds with an at least 10-fold higher affinity to said LLE than to said naturally occurring molecule. a) an anti-linker/label epitope chimeric antigen receptor (anti-LLE CAR) comprising I) an extracellular linker/label epitope (LLE) binding domain, II) a transmembrane domain, and III) a signal transduction domain, wherein said extracellular LLE binding domain binds a target cell binding molecule (TCBM), I) an extracellular linker/label epitope (LLE) binding domain, II) a transmembrane domain, and III) a signal transduction domain, b) said target cell binding molecule (TCBM) comprising i) an antigen binding moiety (ABM), wherein said ABM binds specifically to an antigen expressed on a target cell, ii) a label moiety (LaM), wherein said LaM is a naturally occurring molecule in a subject or a derivative thereof, wherein said naturally occurring molecule is in the circulatory system of said subject, iii) a linker moiety (LiM) conjugating said ABM and said LaM, thereby forming a linker/label epitope (LLE), wherein said LiM is selected from the group consisting of peptides, alkyl chains, alkanoic acids, carboxylic acids, farnesyls, polyethylene glycols, or a derivative thereof, wherein said extracellular LLE binding domain binds said LLE with a higher preference than said naturally occurring molecule, wherein said extracellular LLE binding domain binds with an at least 10-fold higher affinity to said LLE than to said naturally occurring molecule. i) an antigen binding moiety (ABM), wherein said ABM binds specifically to an antigen expressed on a target cell, ii) a label moiety (LaM), wherein said LaM is a naturally occurring molecule in a subject or a derivative thereof, wherein said naturally occurring molecule is in the circulatory system of said subject, iii) a linker moiety (LiM) conjugating said ABM and said LaM, thereby forming a linker/label epitope (LLE), wherein said LiM is selected from the group consisting of peptides, alkyl chains, alkanoic acids, carboxylic acids, farnesyls, polyethylene glycols, or a derivative thereof, <EOS>