As mentioned above, although CS is the major risk factor for COPD, only about 20–30% of chronic smokers are susceptible to the disease, suggesting that susceptibility of smokers to COPD varies [1, 7, 9]. This implies that parameters governing the dynamics of TD as well as other important network elements in the model can change among smokers with different levels of COPD susceptibility. Importantly, effects of cigarette smoking cessation can be different among smokers with variable susceptibility . While quitting smoking can prevent COPD in some patients (i.e., reversible susceptible smokers), smoking cessation fails to slow or stop COPD progression in others (severely susceptible smokers) [2, 66].