A diabody molecule comprising a first polypeptide chain and a second polypeptide chain, wherein: (a) said first polypeptide chain comprises in the N-terminal to C-terminal direction: (i) a first domain comprising a binding region of a light chain variable domain of a first immunoglobulin specific for a first epitope, (ii) a second domain comprising a binding region of a heavy chain variable domain of a second immunoglobulin specific for a second epitope, and (iii) a third domain comprising the amino acid sequence VEPKSC ( SEQ ID NO: 79) or the V215A mutation thereof having the sequence AEPKSC (amino acid residues 245-250 of SEQ ID NO: 18); and wherein said first domain and second domain are covalently linked to one another by a peptide linker of 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 amino acid residues, such that the first domain and second domain do not associate to form an epitope binding site; and (b) said second polypeptide chain comprises in the N-terminal to C-terminal direction: (i) a fourth domain comprising a binding region of a light chain variable domain of the second immunoglobulin (ii) a fifth domain comprising a binding region of a heavy chain variable domain of the first immunoglobulin, and (iii) a sixth domain comprising a cysteine-containing light chain constant domain that comprises the C-terminal 6 amino acid residues of a human light chain constant domain; and wherein: said fourth domain and fifth domain are covalently linked to one another by a peptide of 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 amino acid residues, such that the fourth domain and fifth domain do not associate to form an epitope binding site; said first polypeptide chain and said second polypeptide chain are covalently linked via a disulfide bond between said third domain and said sixth domain; said first domain and said fifth domain associate to form a first binding site that binds said first epitope; said second domain and said fourth domain associate to form a second binding site that binds the second epitope; said first and second epitopes are different; and one of said first and said second binding sites binds to a pathogenic antigen or a cancer antigen; and wherein the diabody molecule is capable of simultaneously binding the first and second epitopes. (a) said first polypeptide chain comprises in the N-terminal to C-terminal direction: (i) a first domain comprising a binding region of a light chain variable domain of a first immunoglobulin specific for a first epitope, (ii) a second domain comprising a binding region of a heavy chain variable domain of a second immunoglobulin specific for a second epitope, and (iii) a third domain comprising the amino acid sequence VEPKSC ( SEQ ID NO: 79) or the V215A mutation thereof having the sequence AEPKSC (amino acid residues 245-250 of SEQ ID NO: 18); and wherein said first domain and second domain are covalently linked to one another by a peptide linker of 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 amino acid residues, such that the first domain and second domain do not associate to form an epitope binding site; and (i) a first domain comprising a binding region of a light chain variable domain of a first immunoglobulin specific for a first epitope, (ii) a second domain comprising a binding region of a heavy chain variable domain of a second immunoglobulin specific for a second epitope, and (iii) a third domain comprising the amino acid sequence VEPKSC ( SEQ ID NO: 79) or the V215A mutation thereof having the sequence AEPKSC (amino acid residues 245-250 of SEQ ID NO: 18); (b) said second polypeptide chain comprises in the N-terminal to C-terminal direction: (i) a fourth domain comprising a binding region of a light chain variable domain of the second immunoglobulin (ii) a fifth domain comprising a binding region of a heavy chain variable domain of the first immunoglobulin, and (iii) a sixth domain comprising a cysteine-containing light chain constant domain that comprises the C-terminal 6 amino acid residues of a human light chain constant domain; (i) a fourth domain comprising a binding region of a light chain variable domain of the second immunoglobulin (ii) a fifth domain comprising a binding region of a heavy chain variable domain of the first immunoglobulin, and (iii) a sixth domain comprising a cysteine-containing light chain constant domain that comprises the C-terminal 6 amino acid residues of a human light chain constant domain; said fourth domain and fifth domain are covalently linked to one another by a peptide of 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 amino acid residues, such that the fourth domain and fifth domain do not associate to form an epitope binding site; said first polypeptide chain and said second polypeptide chain are covalently linked via a disulfide bond between said third domain and said sixth domain; said first domain and said fifth domain associate to form a first binding site that binds said first epitope; said second domain and said fourth domain associate to form a second binding site that binds the second epitope; said first and second epitopes are different; and one of said first and said second binding sites binds to a pathogenic antigen or a cancer antigen;