The second line of immune defense is the collection of cells and fluids that provide a nonspecific reaction against invading pathogens.
If invading pathogens are able to make it past the skin or mucosal membranes, they encounter the second line of defense, a direct, nonspecific response by innate immune cells. Once activated, these systems send out a chemical signal called a cytokine, one of a family of chemical messengers that regulate several different immune functions. Cytokines alarm the surrounding tissue, causing an inflammatory response, initiating the complement system, and attracting white blood cells to stop the infection before it takes hold.Inflammation is a local response to injured tissue that results in an increased blood flow to the affected site and triggers a cell-mediated immune response if the second line of defense fails. It is characterized by swelling, heat, redness, pain, and dysfunction of the surrounding tissue. Swelling is caused by a sudden influx of fluid carried by the blood and the lymphatic system, bringing with it a collection of immune cells that attack any potential pathogens. The heat is caused by inflammatory molecules that signal to local cells to produce and retain more heat, making the tissue less favorable for growth of pathogens. Fever is a more generalized response to infection similar to heat of inflammation. Invading pathogens are not the only cause of inflammation. In certain circumstances the innate immune system can confuse chemicals from food or the air as invading bacteria, causing an improper and potentially dangerous inflammatory response that is called an allergic response.
Human blood is made up of four main components: plasma, erythrocytes, platelets, and leukocytes. Plasma makes up the liquid in which the other components are suspended; the other components are collectively referred to as "formed elements." The erythrocyte, or red blood cell, is the main method of oxygen delivery to the body. Platelets help clot blood. The leukocyte is a type of formed element in blood that aids the immune process and protects the body from infections and foreign invasion. Leukocytes lack hemoglobin and are therefore colorless, or "white," which is why they are also referred to as white blood cells. Several types of leukocytes exist and are all derived from hematopoietic stem cells found in the bone marrow. All leukocytes begin differentiation in bone marrow and most complete maturation in blood or tissues.
Some leukocytes, such as B cells and T cells, are a part of the adaptive immune system, but many are a part of innate immunity, such as basophils, neutrophils, and eosinophils. Leukocytes respond to molecules that appear on the surface of all types of cells using what is known as a pattern recognition receptor, a cellular receptor that binds specifically to molecules commonly associated with pathogens in order to signal infection to leukocytes in an innate immune reaction. These receptors are immune surface proteins that detect whether chemicals are self or nonself. Self refers to molecules detected by the immune system that signal to immune cells the molecule is a part of the host body. Nonself refers to molecules detected by the immune system that signal to immune cells the molecule may be an invading pathogen. Pattern recognition receptors bind to microbe-specific molecules, such as carbohydrates associated with cell walls, peptides associated with flagella, or nucleic acids, that identify them as nonself. A special type of leukocyte for innate immunity is the natural killer (NK) cell. Natural killer cells recognize and respond to cells infected with viruses and some types of cancerous cells. The response of NK cells involves releasing cytotoxic granules that result in destruction of the infected cell through lysis or apoptosis. Another mechanism of innate defense is the complement system, which is a collection of immune system pathways that protect the body from microbes by enhancing the function of leukocytes and antibodies. Innate immunity is also aided by interferon, a class of signaling proteins secreted by host cells when pathogens or tumor cells are detected. Interferons function to regulate immune responses, activate immune cells such as NK cells, and trigger neighboring cells of viral infection allowing them to prepare a response that slows the spread of the viral infection.