• The lymph nodes are the sites where B and T cells respond to antigens that are collected by the lymph draining peripheral tissues. Lymphocytes in the spleen respond to bloodborne antigens. Both lymph nodes and spleen are organized into B cell areas (follicles) and T cell areas (parafollicular zones). The T cell areas are also the sites of residence of mature dendritic cells, which are accessory cells specialized for the activation of naïve T cells. Follicular dendritic cells reside in the B cell areas and serve to activate B cells during humoral immune responses to protein antigens. The development of secondary lymphoid tissue architecture depends on cytokines.
• Accessory cells function to promote the activation of lymphocytes and to perform effector functions that may be enhanced by humoral or cell-mediated adaptive immune responses. Accessory cells include mononuclear phagocytes, dendritic cells, and follicular dendritic cells.
• Follicular • Hematopoietic • Residence • Architecture • Cytokines • Langerhans cell • Gastrointestinal • Tonsil • Recruit • Microbial • Vascular endothelial cells
• Some of the progeny of antigen-activated B and T lymphocytes differentiate into memory cells that survive for long time periods in a quiescent state. These memory cells are responsible for the rapid and enhanced responses to subsequent exposures to antigen.
• The mucosal immune system includes specialized collections of lymphocytes and accessory cells organized to optimize encounter with environmental antigens introduced through the respiratory and gastrointestinal and genitourinary tracts. Peyer’s patches in the intestinal wall and the tonsils in the oropharynx are examples of mucosal immune system tissue.
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• The anatomic organization of the cells and tissues of the immune system is of critical importance for the generation of immune responses. This organization permits the small number of lymphocytes specific for any one antigen to locate and respond effectively to that antigen regardless of where in the body the antigen is introduced.
• B and T lymphocytes express highly diverse and specific antigen receptors and are the cells responsible for the specificity and memory of adaptive immune responses.
• The cutaneous immune system consists of specialized collections of accessory cells and lymphocytes adapted to respond to environmental antigens encountered in the skin. A network of immature dendritic cells called Langerhans cells is present in the epidermis of the skin that serves to trap antigens and then transport them to drainymphocyte recirculation is the process by which lymphocytes continuously move between sites throughout the body through blood and lymphatics, and it is critical for the initiation and effector phases of immune responses. Naïve T cells normally recirculate among the various peripheral lymphoid organs, increasing the likelihood of encounter with antigen displayed by antigen-presenting cells such as mature dendritic cells. Memory and effector T cells more typically are recruited to peripheral sites of inflammation where microbial antigens are located
• The adaptive immune response depends on antigen-specific lymphocytes, accessory cells required for lymphocyte activation, and effector cells that eliminate antigens.
• The organs of the immune system may be divided into the generative organs (bone marrow and thymus), where lymphocytes mature, and the peripheral organs (lymph nodes and spleen), where naïve lymphocytes are activated by antigens.
Lesson Thirteen
Immune System
New words and phrases
• Anatomic • Receptor • Nomenclature • Precursor • Thymus • Naive • Progeny • Quiescent • Humoral • Mononuclear • Phagocyte • Dendritic
• Naïve B and T cells are mature lymphocytes that have not been stimulated by antigen to become differentiated lymphocytes. When they encounter antigen, they differentiate into effector lymphocytes that have functions in protective immune responses. Effector B lymphocytes are antibody-secreting plasma cells. Effector T cells include cytokine-secreting CD4+ helper T cells and CD8+ cytolytic (or cytotoxic) T lymphocytes (CTLs).
• Both B and T lymphocytes arise from a common precursor in the bone marrow. B cell development proceeds in the bone marrow, whereas T cell precursors migrate to and mature in the thymus. After maturing, B and T cells leave the bone marrow and thymus, enter the circulation, and populate peripheral lymphoid organs.