The Blood and
Lymphatic Systems
As mentioned already, many
cells of the immune system, particularly lymphocytes, NK cells, monocytes,
neutrophils, eosinophils, and basophils travel around the body in the blood and
lymph. Both the blood vessels and lymphatic vessels are lined with a type of
epithelial cell referred to as endothelium. As infectious agents
can, collectively, infect any organ or tissue, this motility of the immune
system is essential in order to protect the whole body. Leukocytes are carried
through the blood circulation by the pumping action of the heart, and travel
from the heart through the arteries to eventually reach the capillaries found
throughout the tissues. The leukocytes can continue their journey in the veins,
which contain internal valves to ensure the blood continues to flow in the
correct direction, eventually leading back to the heart. Thus leukocytes can go
around the body again and again via the blood circulation. The system of
lymphatic vessels (Figure 6.10) is also distributed throughout the body and makes physical connections with the blood
circulation in the thorax (the chest). Here a lymphatic vessel called the thoracic
duct (also referred to as the left lymphatic duct) joins up with the
left subclavian vein, while the right lymphatic duct joins to the
right subclavian vein.
Small lymphatic capillaries
collect interstitial fluid (the fluid that surrounds and bathes cells) and join
up with each other to form the afferent lymphatic vessels. The
various motile cells of the immune system, and any pathogens or fragments of
pathogens that might be present, can also be carried with the interstitial
fluid into the afferent lymphatics. The fluid is now referred to as lymph
and moves through the lymphatic vessels because of the peristaltic
activity of the vessels coupled with valves that ensure unidirectional flow.
The afferent lymphatic vessels eventually enter organized lymphoid structures
called lymph nodes. The lymph can subsequently leave the lymph
nodes via the efferent lymphatic vessels, and will eventually mix
with the blood using the connections we have described
earlier.
