MCAT Biology Review

Chapter 8: The Immune System

8.4 The Lymphatic System

The immune system and the lymphatic system are intimately related. B-cells proliferate and develop within the lymphatic system, especially the lymph nodes. This system also serves other necessary functions for the body.

STRUCTURE

The lymphatic system, along with the cardiovascular system, is a type of circulatory system. It is made up of one-way vessels that become larger as they move toward the center of the body. These vessels carry lymphatic fluid (lymph) and join to comprise a large thoracic duct in the posterior chest, which then delivers the fluid into the left subclavian vein (near the heart).

Lymph nodes are small, bean-shaped structures along the lymphatic vessels. Lymph nodes contain a lymphatic channel, as well as an artery and a vein. The lymph nodes provide a space for the cells of the immune system to be exposed to possible pathogens.

REAL WORLD

Certain cancers, especially breast cancer, are prone to spread via lymphatic channels. Mastectomy, a surgery that removes the breast, is one of the treatments for breast cancer. In order to ensure that all of the cancer has been removed, lymph nodes are often removed at the same time.

FUNCTION

The lymphatic system serves many different purposes for the body by providing a secondary system for circulation.

Equalization of Fluid Distribution

At the capillaries, fluid leaves the bloodstream and goes into the tissues. The quantity of fluid that leaves the tissues at the arterial end of the capillary bed depends on both hydrostatic and oncotic pressures (Starling forces). Remember that the oncotic pressure of the blood draws water back into the vessel at the venule end, once hydrostatic pressure has decreased. Because the net pressure drawing fluid in at the venule end is slightly less than the net pressure pushing fluid out at the arterial end, a small amount of fluid remains in the tissues. Lymphatic vessels drain these tissues and subsequently return the fluid to the bloodstream.

The lymphatics offer some protection against pathology. For example, if the blood has a low concentration of albumin (a key plasma protein), the oncotic pressure of the blood is decreased, and less water is driven back into the bloodstream at the venule end. Thus, this fluid will collect in the tissues. Provided that the lymphatic channels are not blocked, much of this fluid may eventually return to the bloodstream via the lymphatics. Only when the lymphatics are overwhelmed does edema occur—swelling due to fluid collecting in tissue.

Transportation of Biomolecules

The lymphatic system also transports fats from the digestive system into the bloodstream. Lacteals, small lymphatic vessels, are located at the center of each villus in the small intestine. Fats, packaged into chylomicrons by intestinal mucosal cells, enter the lacteal for transport. Lymphatic fluid carrying many chylomicrons takes on a milky white appearance and is called chyle.

Immunity

As stated previously in this chapter, lymph nodes are a place for antigen-presenting cells and lymphocytes to interact. B-cells proliferate and mature in the lymph nodes in collections called germinal centers.

MCAT Concept Check 8.4:

Before you move on, assess your understanding of the material with these questions.

1.    Filariasis is the name for an infection with a certain group of parasites, most notably Wuchereria bancrofti. This parasite resides in lymph nodes and causes blockage of flow. If an individual had a W. bancrofti infection in the lymph nodes of his or her thigh, what would likely happen?

2.    What structure is responsible for returning materials from lymphatic circulation to the cardiovascular system?