The second line of defense is made possible through the lymphatic system, a network of vessels, nodes, and organs that acts as a secondary circulation system. Lymph vessels are spread throughout the body, allowing immune cells to reach any location in order to respond to infection. Unlike the circulatory system, where the heart pumps blood through vessels to maintain movement, lymph is circulated by a squeezing action by skeletal muscles and pressure changes in the thoracic cavity.
Connecting the many lymph vessels are hundreds of structures called lymph nodes. Aggregates of lymph nodes are found in the cervical (neck), axillary (armpit), and inguinal (groin) regions. Many types of disease and infection result in swelling of the lymph nodes. Pathogens from infected tissues are brought into lymph nodes via lymphatic vessels; thus lymph nodes serving as filters for lymph fluid. Lymphocytes that scan for signals indicating infection frequently circulate through the lymph nodes. The spleen and liver act as filters for the circulatory system and lymph vessels, destroying old erythrocytes and conducting surveillance for pathogens. Similarly, the tonsils contact incoming air, food, and fluid and are exposed to potential pathogens before the rest of the body. The location of the tonsils enables the activation of an early immune response. Infection of the tonsils is relatively common, particularly with viral respiratory pathogens, and results in inflammation known as tonsillitis.All of these components together allow the lymphatic system to serve three major functions, with some overlap among them. The first function is to remove excess fluids that accumulate in parts of the body, which often occurs as a result of swelling during an inflammatory response. The second function is to absorb and transport lipid molecules to the circulatory system, where they can reach damaged or infected cells. The third major function is the proliferation, development, and delivery of new leukocytes, some of which in turn produce antibodies and generate adaptive immunity.