have a role in the defense of the body against infection. Many of these organs are also part
of the immune system.
Tonsils
The tonsils are areas of lymphoid tissue on either side of the throat. The termtonsilsrefers
most often to the tonsils in the back of the throat as shown inFigure18.9. But, there are
tonsils in the nasal cavity and behind the tongue too. Like other organs of the lymphatic
system, the tonsils are also part of the immune system. The immune system help protect
the body against infection. The tonsils are believed to help fight off nose and throat and
other upper respiratory tract infections such as colds. Tonsils tend to reach their largest size
near puberty, after which they get smaller.Tonsillitisis an infection of the tonsils that can
cause a sore throat and fever.
Figure 18.9: The termtonsilsrefers most often to the tonsils in the back of the throat, but
there are tonsils in the nasal cavity and behind the tongue too. ( 9 )
Bone Marrow
Bone marrow is the tissue found in the middle of bones. The marrow in the large bones of
adults makes new blood cells. Certain white blood cells, calledT-cells,are made in the bone
marrow and move to the thymus to mature. Other white blood cells calledB-cells,move
from the bone marrow to the spleen after they have matured.