**5. Conclusions**

Alharbi and Rambely [26] formulated their model to examine the impact a modern lifestyle has on our health, as well as the impact of switching back from an unhealthy lifestyle to a healthy lifestyle. In this study, our model aims to understand the natural function of the immune system as regards protecting the human body from developing cancer where the progress of abnormal cells might trigger the appearance of tumor cells. Hence, the immune–healthy diet and immune–unhealthy diet models have been studied dynamically, analytically, and numerically. By comparing the results of the analysis and simulation of both the IHDM and IUNHDM, we suggested that there are three stages which act to stop abnormal cells from progressing into tumor cells. In the first stage, the immune system receives a signal which provokes it to recognize abnormal cells as foreign. Next, immune cells are activated to attack the abnormal cells. The results of this interaction lead to the inhibition or elimination of the abnormal cells. Finally, the immune cells typically die after the interaction stage, which indicates that the body is in the recovery stage. These processes were interrupted in the immune–unhealthy diet model, in which the interaction between the immune cells and abnormal cells failed to eliminate the

abnormal cells and also led to a decrease in immune cells. Thereafter, the abnormal cells succeeded in coexisting with the normal cells; this type of coexistence might trigger cancer [5,20,22,24,26,40]. On the basis of the analysis and simulation of the IHDM and the IUNHDM, we can infer that the process of elimination or inhibition of abnormal cells by the immune system is affected by two main parameters, namely, *η* and *μ*. When a person follows a healthy diet (correlating with the food pyramid), his/her immune system will be strong and able to recognize damaged cells, responding by repairing or eliminating them. By contrast, following an unhealthy diet leads to a weakened immune system, which harms its function. As a result, abnormal cells will stimulate a response from the immune system, encouraging it to increase the generation of immune cells. This stimulation increased when associated with the IUNHDM and decreased when associated with IHDM. In summary, symmetry and antisymmetry are basic characteristics in the understanding of the relationship between dietary patterns and the behavioral responses of the immune system when protecting the human body from developing diseases. The symmetry of the IHDM and IUNHDM can be seen when the immune system in both models responds to abnormal cells appearing in the tissue, as well as the responses in the recovery stage. The results also suggested some similarities in terms of the function of the immune system in both models, possibly affected by diet habits. Although the mathematical model that is proposed in this work contributes to understanding the general dynamics of pathogens, it is well-known that mathematical models cannot take all variables into account. For this reason, it is highly recommended to conduct clinical experiments to consider real cases in order to confirm the results of our mathematical model and to show more precise results. In the future, we will expand this work to study the dynamic effect of the growth of abnormal cells and their activity. In addition, we plan to apply our work to other pathogens.

**Author Contributions:** Conceptualization, S.A.A.; Funding Acquisition, A.S.R.; Methodology, S.A.A.; Project Administration, A.S.R.; Supervision, A.S.R.; Validation, A.S.R.; Writing—Original Draft, S.A.A.; Writing—Review and Editing, A.S.R.

**Funding:** This research is funded by a gran<sup>t</sup> from Universiti Kebangsaan Malaysia.

**Acknowledgments:** We are indebted to Universiti Kebangsaan Malaysia for providing financial support and facilities for this research under the gran<sup>t</sup> GUP-2017-112.

**Conflicts of Interest:** The authors declare no conflict of interest.
