Polypeptide comprising a first and a second immunoglobulin single variable domain, wherein - said first ISV has high affinity for/binds to cluster of differentiation 3 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 CD3; wherein said target cell is different from said T cell; and 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: 81-87; 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: 81; and/or (ii) CDR2 is chosen from the group consisting of: (c) SEQ ID NOs: 101-109; 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: 101; and/or (iii) CDR3 is chosen from the group consisting of: (e) SEQ ID NOs: 123-127; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123. - said first ISV has high affinity for/binds to cluster of differentiation 3 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 CD3; wherein said target cell is different from said T cell; and 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: 81-87; 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: 81; and/or (ii) CDR2 is chosen from the group consisting of: (c) SEQ ID NOs: 101-109; 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: 101; and/or (iii) CDR3 is chosen from the group consisting of: (e) SEQ ID NOs: 123-127; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123. (i) CDR1 is chosen from the group consisting of: (a) SEQ ID NOs: 81-87; 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: 81; and/or (a) SEQ ID NOs: 81-87; 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: 81; and/or (ii) CDR2 is chosen from the group consisting of: (c) SEQ ID NOs: 101-109; 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: 101; and/or (c) SEQ ID NOs: 101-109; 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: 101; and/or (iii) CDR3 is chosen from the group consisting of: (e) SEQ ID NOs: 123-127; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123. (e) SEQ ID NOs: 123-127; and (f) amino acid sequences that have 3, 2, or 1 amino acid(s) difference with the amino acid sequence of SEQ ID NO: 123.