An immune effector cell comprising a) an antigen binding receptor, wherein said antigen binding receptor binds to a tag of a tagged polypeptide, and wherein the polypeptide of said tagged polypeptide binds to an antigen expressed on the surface of a target cell to be killed, and wherein the binding of said antigen binding receptor to said tag activates and stimulates an immune response of said immune effector cell thereby leading to cytotoxic activity and cytokine release of said immune effector cell, wherein said antigen binding receptor is a chimeric antigen receptor (CAR) comprising an antigen binding domain, a transmembrane domain and a cytoplasmic signaling domain comprising at least one primary cytoplasmic signaling domain and at least one co-stimulatory signaling domain, wherein said antigen binding domain of said CAR binds to the tag of said tagged polypeptide, and b) a chimeric costimulatory receptor (CCR), wherein said CCR binds to a further antigen, wherein said further antigen is not expressed on the surface of said target cell, and wherein the binding of said CCR to said further antigen boosts said immune response of said immune effector cell and/or ensures survival of said immune effector cell, wherein said CCR comprises an antigen binding domain, a transmembrane domain and a cytoplasmic part comprising at least one co-stimulatory domain, wherein said CCR does not comprise CD3ζ, and wherein said antigen binding domain of said CCR binds to said further antigen, wherein said CCR is not able to mediate said immune response on its own, and wherein the magnitude of enhancement of the effector response of the immune cell expressing both the CAR, and the CCR is at least 1.5 fold compared to the effector response of the immune cell that is activated and stimulated solely by the CAR, and that is not boosted by the CCR. a) an antigen binding receptor, wherein said antigen binding receptor binds to a tag of a tagged polypeptide, and wherein the polypeptide of said tagged polypeptide binds to an antigen expressed on the surface of a target cell to be killed, and wherein the binding of said antigen binding receptor to said tag activates and stimulates an immune response of said immune effector cell thereby leading to cytotoxic activity and cytokine release of said immune effector cell, wherein said antigen binding receptor is a chimeric antigen receptor (CAR) comprising an antigen binding domain, a transmembrane domain and a cytoplasmic signaling domain comprising at least one primary cytoplasmic signaling domain and at least one co-stimulatory signaling domain, wherein said antigen binding domain of said CAR binds to the tag of said tagged polypeptide, and b) a chimeric costimulatory receptor (CCR), wherein said CCR binds to a further antigen, wherein said further antigen is not expressed on the surface of said target cell, and wherein the binding of said CCR to said further antigen boosts said immune response of said immune effector cell and/or ensures survival of said immune effector cell, wherein said CCR comprises an antigen binding domain, a transmembrane domain and a cytoplasmic part comprising at least one co-stimulatory domain, wherein said CCR does not comprise CD3ζ, and wherein said antigen binding domain of said CCR binds to said further antigen, wherein said CCR is not able to mediate said immune response on its own, and wherein the magnitude of enhancement of the effector response of the immune cell expressing both the CAR, and the CCR is at least 1.5 fold compared to the effector response of the immune cell that is activated and stimulated solely by the CAR, and that is not boosted by the CCR. <EOS>