A multicellular organism has five levels of cellular organization. At the very base are the cells—the basic units of all living things. Then come the tissues, which consist of groups of cells; then the organs, which consist of several groups of tissues; then the organ or body systems, which control all bodily functions within the living organism; and finally the organism itself.

The human body has eleven organ systems, which consist of various cells, tissues, and organs that work together to make sure that the organism stays alive and well.

One of these body systems is the respiratory system. The three main types of cells that make up the respiratory system are the cilia cells, which are located on the bronchi and trachea, make sure that the airways are clean, and help move mucus out of the lungs; the goblet cells, which secrete mucus, trap various pathogens, and make sure that the respiratory system is functioning properly; and the basal cells, which respond to injuries.

The tissues within the respiratory system are the respiratory mucosa (epithelium and supporting lamina propria, as well as the pleura of the lungs), submuscosa, connective tissue (cartilage and/or muscle layer), and adventitia. The organs of the respiratory system are the nose and mouth, pharynx, larynx, trachea, bronchi, and lungs.

The immune system consists of two main types of cells: lymphocytes (T-cells, B-cells, and NK cells) and phagocytes (neutrophils, monocytes/macrophages, and mast cells), which are all white blood cells and help the organism fight off various diseases and pathogens. The two main types of tissues and organs found in the immune system are the primary lymphoid tissues and organs, which include the thymus gland, the bone marrow, the secondary lymphoid tissues, and the organs, which include the spleen, the tonsils, the lymph nodes, the Peyer's patches, and the mucosa-associated lymphoid tissue (MALT).