The associated immune system is highly complicated as it involves many mediator molecules, multiple immune cell types, and lung tissues, presenting a challenge for quantifying the dynamics of CS-induced COPD. For simplicity in the following discussion, an immune response network model is constructed by treating important immune cells, cytokines, and lung tissues as network nodes. There are two types of inputs initiated from a node: a positive or an up-regulation input (denoted by→) indicates that an increasing of the concentration of the tail node will lead to an increasing of that of the head node or an up-regulation of the process when the input arrow ends at an edge between two nodes, and vice versa for a negative or a down-regulation (inhibition) input (denoted by ┫).