Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
reaction [riːˈækʃən] n (CHEM) реа́кция; (also MED, POL)
reaction (to/against) реа́кция (на +acc/про́тив +gen)
reactions
npl (reflexes) реа́кция fsg
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.