Context: If pathogens get through the bodys first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. It is called an immune response, and is a specific type of response. The immune system has a special response for each type of pathogen. The immune system ( Figure 1.1) is also part of the lymphatic systemnamed for lymphocytes, which are the type of white blood cells involved in an immune response. They include several lymph organs, lymph vessels, lymph, and lymph nodes. This diagram shows the parts of the im- mune system. The immune system in- cludes several organs and a system of vessels that carry lymph. Lymph nodes are located along the lymph vessels. The lymph organs are the red bone marrow, tonsils, spleen, and thymus gland. They are described below ( Figure Each lymph organ has a different job in the immune system. Lymph vessels make up a circulatory system that is similar to the cardiovascular system, which you can read about in a previous concept. Lymph vessels are like blood vessels, except they move lymph instead of blood. Lymph is a yellowish liquid that leaks out of tiny blood vessels into spaces between cells in tissues. Where there is more inflammation, there is usually more lymph in tissues. This lymph may contain many pathogens. The lymph that collects in tissues slowly passes into tiny lymph vessels. It then travels from smaller to larger lymph vessels. Lymph is not pumped through lymph vessels like blood is pumped through blood vessels by the heart. Instead, muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. The lymph finally reaches the main lymph vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph returns to the bloodstream. Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes. Lymph nodes are small, oval structures located along the lymph vessels. They act like filters. Any pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes. Lymphocytes ( Figure 1.3), a type of white blood cell, are the key cells of an immune response. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the bodys lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs. There are two main types of lymphocytes: 1. B cells. This image of a lymphocyte was made with an electron microscope. The lym- phocyte is shown 10,000 times its actual size. 2. T cells. Both types of lymphocytes are produced in the red bone marrow. They are named for the sites where they grow larger. The "B" in B cells stands for bone. B cells grow larger in red bone marrow. The "T" in T cells stands for thymus. T cells mature in the thymus gland. B and T cells must be switched on in order to fight a specific pathogen. Once this happens, they produce an army of cells ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have proteins, often located on their cell surface. These proteins are called antigens. An antigen is any protein that causes an immune response, because it is unlike any protein that the body makes. Antigens are found on bacteria, viruses, and other pathogens. Your body sees these as foreign, meaning they do not belong in your body. 
 Question: t cells mature in the
Output: thymus gland.