15-4 The immune system is a complex collection of organs, tissues, and cells

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• The primary tissues of the immune system are the bone marrow and thymus and these are involved in the creation and maturation of immune cells.
• The secondary tissues of the immune system are the spleen, lymphatic system, lymph nodes and MALT. These tissues contain mature immune cells and are the active part of the immune system.

The immune system consists of a complex network of organs and tissues, connected by blood and lymphatic vessels, that work together to prevent infection. Many of these systems are unleashed by activation of other parts of the immune system. The extensive interdependence of the players of the immune system can make it difficult for the beginning student to understand. As we go through the immune system, you will encounter unfamiliar terms, but be patient because they will be explained later in the chapter. To try to make it a bit easier, we will first describe the anatomy of the immune system, then examine the various cell types involved in immunity, and finish this section by describing how these parts work together in reacting to an invading pathogen.

Many different organs and tissues in the body contribute to the function of the immune system. These include the circulatory system, bone marrow, thymus, spleen, lymphatic system and Mucosal Associated Lymphoid Tissue (MALT). Together these tissues are responsible for the creation, transport and successful operation of mammalian immunity. In this section we will look at the role of each tissue and introduce a number of cells and proteins involved in immunity.

The tissues of the immune system fall into two groups based upon their role in host defense. Primary (or central) tissues look after immature immune cells, creating and educating them during their differentiation into mature cells. The bone marrow and thymus gland are parts of the primary immune system. Secondary (peripheral) immune organs look after mature cells that are an active part of defense. The secondary system encompasses the rest of the immune tissues: the spleen, the lymphatic system, lymph nodes and MALT. Of course it is not this simple and the spleen and MALT also help in the maturation of immune cells. Figure 15-2 shows the anatomy of the blood and lymph systems.

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