Compounds of formula (IA) or (IB) are inhibitors of CD80 and useful in immunomodulation therapy: wherein Ar represents an optionally substituted monocyclic or bicyclic aromatic or heteroaromatic group having from 5 to 10 ring atoms; R1 and R2 independently represent H, or C1-C6 alkyl; R3 represents H; F; CI; Br, —NO2; —CN; C1-C6 alkyl optionally substituted by F or CI; or C1-C6 alkoxy optionally substituted by F; R4 represents a carboxylic acid group (—COOH) or an ester thereof, or —C(═O)NR6R7, —NR7C(═O)R6, —NR7C(═O)OR6, —NHC(═O)NR7R6 or —NHC(═S)NR7R6 wherein R6 represents H, or a radical of formula -(Alk)m-Q wherein m is 0 or 1, Alk is an optionally substituted divalent straight or branched C1-C12 alkylene, or C2-C12 alkenylene, or C2-C12 alkynylene radical or a divalent C3-C12 carbocyclic radical, any of which radicals may be interrupted by one or more —O—, —S— or —N(R8)— radicals wherein R8 represents H or C1-C4 alkyl, C3-C4 alkenyl, C3-C4 alkynyl, or C3-C6 cycloalkyl, and Q represents H; —CF3; —OH; —SH; —NR8R8 wherein each R8 may be the same or different, or form a ring when taken together with the nitrogen to which they are attached; an ester group; or an optionally substituted aryl, aryloxy, cycloalkyl, cycloalkenyl or heterocyclic group; and R7 represents H or C1-C6 alkyl; or when taken together with the atom or atoms to which they are attached R6 and R7 form a monocyclic heterocyclic ring having 5, 6 or 7 ring atoms; and X represents a bond or a divalent radical of formula -(Z)n-(Alk)- or -(Alk)-(Z)n- wherein Z represents —O—, —S— or —NH—, Alk is as defined in relation to R6 and n is 0 or 1.