Table 1.
Characteristics of complex systems
| Characteristic | Description | Example |
|---|---|---|
| Multicomponent | Complex systems involve multiple components (also “agents” or “actors”), which are essential to define how the system works5,6 | Lymphocytes, neutrophils, monocytes, and macrophages form together the immune system with many other components |
| Network structure | The actors are interconnected in a network structure in the system51 | The human brain is made up of a complex network of neurons |
| Multi-scaling | In general, complex systems are made up of many interacting complex sub-systems which are themselves built upon multiple complex sub-systems (and so on) | The human body is a complex system that is made up of many complex sub-systems such as organs or tissues which are in turn made of complex systems (cells). These systems interact together at micro-, meso-, and macro scales to appear as a complex system operating at a higher level |
| Out of equilibrium systems | A complex system is generally robust and tends toward balance and homeostasis. Exhibits multiple stable states where the system can reside67 | Inflammation after an infection: systemic response increases, after a while decreases/stabilizes again at a new set point |
| Adaptiveness | Many biological, living systems show an adaptive behavior5,6 | In the human body, most systems are adaptive like the immune system and the neural network. People show adaptive behavior at individual level. In society, we can see adaptive behavior as well. These systems are, from an evolutionary point of view, a logical development because they can adapt their behavior to changes in the environment and are “adaptive” or “learning.” Hence, the name complex adaptive system (CAS) |
| Unpredictability | Unpredictability and uncertainty are normal features of complex systems67 | After a severe burn injury, many events can occur that are fairly unpredictable, such as the occurrence of pneumonia, thrombosis, wound infections, problematic scarring, psychological and social consequences, and so on |
| Nonlinearity | Nonlinearity means that the initial effect of a stressor on the complex system can be small, but the ultimate effect is enormous | The systematic hyperinflammatory response (burn shock) |
| Emergent | Many complex systems may also show emergent behavior: the arising of novel and coherent structures, patterns, or properties during the process of self-organization in complex systems68 | Life itself may be regarded as an example of emergence: the sum of loose components is much bigger than the parts through the interaction of the components of the system. Innovation and creative processes are often the results of emergence |
| Tipping points | In (biological) complex systems are in balance but also there a moment may arise when the state of the system suddenly changes. This is called the tipping point. If and when a tipping point occurs is fairly unpredictable69,70 | After a severe burn injury, the patient suddenly may become hemodynamically unstable |
| Randomness | A complex system can produce outcome without known and predicable order6 | The immune system employs randomness to allow each individual lymphocyte to recognize a range of shapes that differs from the range recognized by other lymphocytes in the population |
| Path dependency | A change in state of system may occur through a certain path, however, the change cannot be reversed following the same path in reverse71 | The process by which a burn turns into scar tissue |