Red Bone Marrow Red bone marrow is found inside many bones, including the femur
shown here. Red bone marrow produces lymphocytes.
Thymus Gland The thymus gland is in the chest behind the breast bone. It stores lym-
phocytes while they mature (Figure21.23).
Figure 21.23: ( 11 )Spleen The spleen is in the abdomen below the lungs. Its job is to filter the blood. Any
pathogens that are filtered out of the blood are destroyed by lymphocytes in the spleen
(Figure21.24).
Tonsils The tonsils are in the throat. They trap pathogens that enter the body through
the mouth or nose. Lymphocytes in the tonsils destroy the trapped pathogens (Figure
21.25).
Lymph and Lymph Vessels
Lymph vessels make up a circulatory system that is similar to the cardiovascular system,
which you read about in the Cardiovascular System chapter. Lymph vessels are like blood
vessels, except they circulate lymph instead of blood.Lymphis a yellowish fluid that leaks
out of tiny blood vessels into spaces between cells in tissues. At sites of inflammation, there
is usually more lymph in tissues. This lymph may contain many pathogens.
The lymph that collects in tissues gradually 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 surrounding the lymph
vessels contract and squeeze the lymph through the vessels. The lymph vessels themselves