Targeting checkpoint proteins expressed on surfaces of T cells as a consequence of sustained tumour antigen stimulation leads to reinvigoration of immune responses and durable clinical benefits in cancer patients. However, the therapeutic interventionsusing checkpoint inhibitors like anti-CTLA4 and anti-PD1 produce different outcomes in different individuals as a consequence of complex interactions between tumour antigens, immune cells and immune responses. Here a discrete time mathematical model is proposed for establishing the quantitative underpinningsof the tumour-immune interplay occurring during tumour growth. The mathematical motif succintlydescribes the tumour growth history with time, the dynamics of tumour antigens and effector T cells and emphasizes on the effect of primary immune responses alone and combination of primary and secondary immune responses on tumour regression. The model compares the tumour growth dynamics observed in persons lacking secondary immune responses with that observed in persons with both immune responses. The anti-tumour immune responses comprising the primary and secondary responses are downregulated by the checkpoint proteins like PD1 and CTLA4 upon continuous stimulation by tumour antigens. The anti-PD1 and anti-CTLA4 inhibitors inhibit the checkpoint proteins and re-strengthen the immune responses resulting in sustained tumour growth or elimination of the tumour. The model developed further forecasts the effect of variation in the strength of immune responses on tumour growth dynamics, and the various outcomes observed when the monotherapy or combination therapy is discontinued after a certain period of time. Further, the model predictions are used to explore the synergistic interactions between the inhibitors and report the maximum synergy achievable for the drug combination.