Which of the following is NOT an organ system in the body?

The cellular system is not classified as one of the traditional organ systems in the human body. Organ systems consist of groups of organs that work together to perform specific functions necessary for the organism's survival. For example, the excretory system manages the removal of waste products, the circulatory system is responsible for the transportation of blood and nutrients, and the nervous system controls and coordinates bodily functions through signaling.

In contrast, the term "cellular system" does not refer to a specific grouping of organs or their functions. While cells are the basic units of life and are essential to all biological processes, the concept of a 'cellular system' lacks recognition as a distinct system like the others listed. Organ systems focus on larger structures and their organized functions, whereas individual cells operate at a micro-level and are part of various systems rather than constituting one themselves.