A compound of formula or a pharmaceutically acceptable salt thereof: wherein L represents a binding moiety selected from an antibody or antigen binding fragment thereof; S 1 represents a spacer selected from a -(CH 2) a - or -(CH 2) b -(CH 2 -CH 2 -O) c -(CH 2) d - group, wherein one to ten of said -CH 2 - groups may optionally be substituted by one or more groups selected from -O-,-S-, =N -, -C(=O)-, -C NH-, -NHC -, cyclohexyl or pyrrolidine-2,5-dione; a represents an integer selected from 1 to 35; b represents an integer selected from 0 to 5; c represents an integer selected from 1 to 20; d represents an integer selected from 1 to 20; S 2 represents a spacer selected from a -(CH 2) e - or -(CH 2) f -(CH 2 -CH 2 -O) g -(CH 2) h - group, wherein one to three of said -CH 2 - groups may optionally be substituted by one or more groups selected from -N -, -C NH- and -NHC -; e represents an integer selected from 1 to 15; f represents an integer selected from 1 to 10; g represents an integer selected from 1 to 20; h represents an integer selected from 1 to 5; z represents an integer selected from 1 to 30; X 1 represents an antibody or antigen binding fragment attachment moiety; Y 1 and Y 2 independently represent a bond, -O-, -S-, -NH-, -NHC -, -C NH-, -OC -, - C O-, -SC -, -C S-, -NHSO 2 -, -SO 2 NH- or -NHC NH- group; F represents a carbohydrate molecule capable of binding to a human anti-alpha-galactosyl antibody; m represents an integer selected from 1 to 5; and Cy represents phenyl, biphenyl, triphenyl, such that when Cy represents biphenyl or triphenyl said -Y 1 -S 1 -X 1 -L group may be present on any of said phenyl rings and said [F-S 2 -Y 2] m - group or groups may be present on any of said phenyl rings. wherein L represents a binding moiety selected from an antibody or antigen binding fragment thereof; S 1 represents a spacer selected from a -(CH 2) a - or -(CH 2) b -(CH 2 -CH 2 -O) c -(CH 2) d - group, wherein one to ten of said -CH 2 - groups may optionally be substituted by one or more groups selected from -O-,-S-, =N -, -C(=O)-, -C NH-, -NHC -, cyclohexyl or pyrrolidine-2,5-dione; a represents an integer selected from 1 to 35; b represents an integer selected from 0 to 5; c represents an integer selected from 1 to 20; d represents an integer selected from 1 to 20; S 2 represents a spacer selected from a -(CH 2) e - or -(CH 2) f -(CH 2 -CH 2 -O) g -(CH 2) h - group, wherein one to three of said -CH 2 - groups may optionally be substituted by one or more groups selected from -N -, -C NH- and -NHC -; e represents an integer selected from 1 to 15; f represents an integer selected from 1 to 10; g represents an integer selected from 1 to 20; h represents an integer selected from 1 to 5; z represents an integer selected from 1 to 30; X 1 represents an antibody or antigen binding fragment attachment moiety; Y 1 and Y 2 independently represent a bond, -O-, -S-, -NH-, -NHC -, -C NH-, -OC -, - C O-, -SC -, -C S-, -NHSO 2 -, -SO 2 NH- or -NHC NH- group; F represents a carbohydrate molecule capable of binding to a human anti-alpha-galactosyl antibody; m represents an integer selected from 1 to 5; and Cy represents phenyl, biphenyl, triphenyl, such that when Cy represents biphenyl or triphenyl said -Y 1 -S 1 -X 1 -L group may be present on any of said phenyl rings and said [F-S 2 -Y 2] m - group or groups may be present on any of said phenyl rings.