Immune System Components PDF

**Immune System Components** Introduction The immune system is responsible for protecting the body from pathogens such as viruses, bacteria, fungi, and parasites, as well as from defective body cells (eg, cancer cells) and toxins. The immune system is composed of cells that carry out the immune re...

**Immune System Components** Introduction The immune system is responsible for protecting the body from pathogens such as viruses, bacteria, fungi, and parasites, as well as from defective body cells (eg, cancer cells) and toxins. The immune system is composed of cells that carry out the immune response and lymphoid organs and tissues, structures in which immune system cells form, mature, and interact with pathogens. This lesson details these immune system components and their locations in the body. 20.1.01 Cells of the Immune System **Leukocytes** are white blood cells responsible for producing an [immune response](javascript:void(0)) to defend the body. Various types of leukocytes exist, and they reside in different parts of the body, including in blood and body tissues. Table 20.1 summarizes characteristics of leukocytes in the immune system. **Table 20.1** Characteristics of immune system cells. Immune cells can be categorized based on the structure of their cytoplasm. Some types of leukocytes (eg, neutrophils, eosinophils, basophils) have protein-containing cytoplasmic granules visible in stained cell preparations. Such leukocytes are referred to as **granulocytes**. Other types of leukocytes (eg, lymphocytes, monocytes) lack visible cytoplasmic granules and are called **agranulocytes**. A table with different types of cells Description automatically generated Chapter 20: Immune System 611 20.1.02 Bone Marrow The **red bone marrow** is a soft, spongy **lymphoid organ** located within bones that gives rise to all types of blood cells, including leukocytes involved in the immune response (see Figure 20.1). Cells originating in the bone marrow begin as multipotent **hematopoietic stem cells**, which can [differentiate](javascript:void(0)) into **lymphoid progenitor cells** or **myeloid progenitor cells**. Lymphoid progenitor cells go on to become lymphocytes, including B lymphocytes (B cells), T lymphocytes (T cells), and natural killer (NK) cells. Most remaining cells of the immune system, including monocytes, macrophages, neutrophils, eosinophils, basophils, mast cells, and some types of dendritic cells, are derived from myeloid progenitor cells. Myeloid progenitor cells also give rise to cells not directly involved in immunity, including [erythrocytes](javascript:void(0)) (red blood cells) and platelet-forming megakaryocytes. **Figure 20.1** Cells derived from hematopoietic stems cells in red bone marrow. In addition to producing all blood cells, the bone marrow functions in lymphocyte maturation. Specifically, B cells begin their maturation process in the bone marrow before migrating to other lymphoid organs, such as the spleen and lymph nodes, where B cell maturation is completed. Bone marrow is discussed further in Concept 18.1.02.  Chapter 20: Immune System 612 20.1.03 Thymus The **thymus** is a lymphoid organ located just superior to the heart, between the lungs (Figure 20.2). The thymus, which is most active prior to puberty, achieves its largest size during adolescence and gradually shrinks thereafter, being replaced over time by adipose tissue and fibrous connective tissue. **Figure 20.2** Location of the thymus. During embryonic development, a population of lymphoid progenitor cells that ultimately differentiate into functional T cells migrates from the bone marrow to the thymus, the site of T cell maturation. During this maturation process, self-reactive T cells that could potentially mount an immune response against the body\'s own healthy cells are inactivated or induced to undergo [apoptosis](javascript:void(0)) (ie, programmed cell death), as shown in Figure 20.3. Consequently, the remaining T cells typically exhibit **self-tolerance**, that is, they do not recognize the body\'s own healthy cells as foreign and therefore do not attack these healthy body cells. A diagram of a person\'s body Description automatically generated Chapter 20: Immune System 613 **Figure 20.3** Elimination of self-reactive T cells in the thymus. 20.1.04 Lymph Node **Lymph nodes** are organs of the [lymphatic system](javascript:void(0)), which is described in more detail in Lesson 13.2. As depicted in Figure 20.4, each lymph node contains lymphoid tissue and is enclosed in a dense fibrous capsule. Lymph nodes are clustered along lymph vessels and filter [lymph](javascript:void(0)) as it flows through the lymphatic system. Lymph nodes contain macrophages that identify pathogens and other foreign materials in the lymph and destroy these materials via [phagocytosis](javascript:void(0)). In addition, dendritic cells migrate to lymph nodes after  Chapter 20: Immune System 614 engulfing pathogens in other tissues. These dendritic cells then participate in the activation of B and T lymphocytes in the lymph nodes. **Figure 20.4** Structure of a lymph node. 20.1.05 Spleen The **spleen**, located in the left upper quadrant of the abdomen (see Figure 20.5), is the largest lymphoid organ. Like lymph nodes, the spleen contains lymphoid tissue surrounded by a capsule and functions in the filtration of a body fluid. However, in contrast to lymph nodes, the spleen filters blood rather than lymph. Within the spleen, there are two distinguishable components: red pulp and white pulp. The red pulp contains red blood cells and macrophages. These macrophages engulf and remove foreign material and pathogens from the blood, while also removing old, abnormal, or damaged red blood cells from circulation. The white pulp of the spleen contains abundant B and T lymphocytes that carry out surveillance of the blood to identify bloodborne pathogens and mount specific immune defenses against them. A diagram of lymphomas and lymphoma cells Description automatically generated Chapter 20: Immune System 615 **Figure 20.5** Structure of the spleen.