15-5 The circulatory system transports the components of the immune system

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• The circulatory system, while not actively involved in the immune defense, serves as a highway for the transport of immune cells.

The circulatory system is responsible for the transport of blood throughout the body and consists of the heart (a pump), the lungs (a gas exchanger) and the vascular system of arteries, capillaries and veins (plumbing). Blood, which runs through this vascular system, contains both cellular and non-cellular components. The major cell type found in blood are red blood cells, whose role is to transport oxygen and carbon dioxide into and out of the body respectively. The circulatory system also plays a secondary role as one of the routes immune cells use for transport around the body. Blood contains many types of what are called white blood cells, which are made up of mainly neutrophils, but also monocytes, T-lymphocytes and B-lymphocytes.

The non-cellular portion of the blood is a liquid called plasma A major constituent of plasma is immunoglobulins (antibodies). It also contains various proteins of the complement system, which are equally important in the immune system. Plasma also contains the protein fibrinogen as a major component. Under the right conditions, fibrinogen participates in a complex series of reactions that eventually result in the formation of a fibrin clot in the blood. Platelets also participate in this process. Blood clots are important in stopping bleeding and also in inhibiting the invasion of advancing pathogens by entrapping them.

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