Let's begin by reviewing the levels of cell organization. At the bottom of the ladder, we have cells themselves, the basic units of life and function. Next up comes tissues, which are groups of cells that work together to perform a function. On the step up from that are organs, which contain various types of tissue and perform many necessary tasks in the body. The heart, lungs, and stomach are examples of organs. Then we have organ systems. Humans have eleven of these systems, in which cells, tissues, and organs all work together to perform tasks. Finally, at the very top is the organism itself, the living thing.

Now let's choose two body systems and see what kinds of cells, tissues, and organs are within them. We'll look first at the immune system, which helps our bodies fight disease. Cells in this system include leukocytes, neutrophils, and lymphocytes, which work to attack pathogens in various ways. Tissues include bone marrow (which makes blood cells, including lymphocytes) and lymphoid tissues (like our tonsils). Organs include the spleen (which contains white blood cells) and the thymus.

The digestive system features cells like epithelial cells, goblet cells, and enteroendocrine cells that make mucus and hormones that aid digestion. Tissues include the mucosa, the submucosa, the muscularis, and the serosa, all of which line the alimentary canal, which moves food through the digestive system. Organs in the system include the stomach and the intestines.