Cytokines are a broad and loose category of small proteins that are important in cell signaling. Their release has an effect on the behavior of cells around them. It can be due to immune response or inflammation. Interferons are a type of cytokine. They are released by host cells in response to the presence of pathogens such as viruses, bacteria, parasites, or tumor cells. They allow for communication between cells to trigger the protective defenses of the immune system that eradicate pathogens or tumors. (B) Cellular barriers - (III) Neutrophils and Macrophages Cellular barriers are formed by immune cells that prevent the spread and movement of foreign substances in the body. Neutrophils and macrophages are types of white blood cells and are part of the immune system. Neutrophils are the most abundant type of granulocytes and the most abundant type of white blood cells in most mammals. They form an essential part of the innate immune system. Their functions vary from the destruction of bacteria and fungi to the stimulation of other immune cells. Macrophages, on the other hand, are a type of white blood cell that engulfs and digests cellular debris, foreign substances, microbes, cancer cells, and anything else that does not have the type of proteins specific to healthy body cells on its surface in a process called phagocytosis. (C) Physiological barriers - (IV) Tears and Saliva Physiological barriers are those which use the body’s own natural processes to prevent infection. For instance, both tears and saliva contain enzymes such as lysozyme which help to break down the cell walls of many kinds of bacteria. (D) Physical barriers - (I) Mucus coating of the respiratory tract Physical barriers are the body's first line of defense against pathogens. These barriers are structural in nature. The mucus coating of the respiratory tract is a great example. It traps pathogens, dust, and other particles that enter the nasal passage with each breath, and because the mucus is constantly moved by cilia, it is eventually coughed or sneezed out, or swallowed and destroyed by stomach acids. Thus, it acts as a physical barrier, preventing pathogens from reaching the lungs.