The line shape of some of the optical d-d transitions in MnI2 and MnBr2 is asymmetric and depends strongly on temperature and magnetic field strength. It is shown that these effects are due to the hopping of excitons from one Mn2+ ion to another. The hopping integral depends strongly on the correlation between the directions of the spins of the two ions involved. The line shape in the magnetically ordered crystals depends in a sensitive way on the type of magnetic order. We also present a simple method to calculate the line shape in crystals where the magnetic moments are not fully ordered.