  We model the interaction between the immune system and tumor cells including
a time delay to simulate the time needed by the latter to develop a chemical
and cell mediated response to the presence of the tumor. The results are
compared with those of a previous paper, concluding that the delay introduces
new instabilities in the system leading to an uncontrolable growth of the
tumour. Then a cytokine based periodic immunotherapy treatment is included in
the model and the effects of its dossage are studied for the case of a weak
immune system and a growing tumour. We find the existence of metastable states
(that may last for tens of years) induced by the treatment, and also of
potentially adverse effects of the dossage frequency on the stabilization of
the tumour. These two effects depend on the delay, the cytokine dose burden and
other parameters considered in the model.
