Polypeptide comprising a first and a second immunoglobulin single variable domain, wherein - said first ISV has high affinity for/binds to the constant domain of the T cell receptor present on a T cell; - said second ISV has high affinity for/binds to a first antigen on a target cell; wherein said first antigen is different from said TCR; and wherein said target cell is different from said T cell, wherein said first ISV essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: (i) CDR1 is chosen from the group consisting of: (a) SEQ ID NOs: 119-123, 125-127, 129, 132 and 133; and (b) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123; and/or (ii) CDR2 is chosen from the group consisting of: (c) SEQ ID NOs: 134-141, 143-144, 146-156, 159-163; and (d) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 153; and/or (iii) CDR3 is chosen from the group consisting of: (e) SEQ ID NOs: 164-166, 169-171, 173-174; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 170. - said first ISV has high affinity for/binds to the constant domain of the T cell receptor present on a T cell; - said second ISV has high affinity for/binds to a first antigen on a target cell; wherein said first antigen is different from said TCR; and wherein said target cell is different from said T cell, wherein said first ISV essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: (i) CDR1 is chosen from the group consisting of: (a) SEQ ID NOs: 119-123, 125-127, 129, 132 and 133; and (b) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123; and/or (ii) CDR2 is chosen from the group consisting of: (c) SEQ ID NOs: 134-141, 143-144, 146-156, 159-163; and (d) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 153; and/or (iii) CDR3 is chosen from the group consisting of: (e) SEQ ID NOs: 164-166, 169-171, 173-174; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 170. (i) CDR1 is chosen from the group consisting of: (a) SEQ ID NOs: 119-123, 125-127, 129, 132 and 133; and (b) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123; and/or (a) SEQ ID NOs: 119-123, 125-127, 129, 132 and 133; and (b) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123; and/or (ii) CDR2 is chosen from the group consisting of: (c) SEQ ID NOs: 134-141, 143-144, 146-156, 159-163; and (d) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 153; and/or (c) SEQ ID NOs: 134-141, 143-144, 146-156, 159-163; and (d) amino acid sequences that have 4, 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 153; and/or (iii) CDR3 is chosen from the group consisting of: (e) SEQ ID NOs: 164-166, 169-171, 173-174; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 170. (e) SEQ ID NOs: 164-166, 169-171, 173-174; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 170.