"Purely linaer" models do not allow for that. If you want to model an interaction using a particular case of the General Linear Model (do not mistake this for a Generalized Linear Model), you have to introduce an artificial extra variable like the product of the two interacting ones.
This new model is still linear with regards to its parameters (this is what matters for getting the estimators), but it is no longer linaer with regards to its variables (you can no longer talk about a linear relationship between regressors and target)