Porth's Pathophysiology, 9e

Chapter 13 Innate and Adaptive Immunity    303

organs. Lymphocytes are produced and undergo ­maturation in the central lymphoid organs (bone marrow and ­thymus) and are subsequently stored in the peripheral lymphoid structures (lymph nodes, spleen, mucosa-­associated lym- phoid tissues in the respiratory, gastrointestinal, and reproductive systems) where they function to concentrate antigen, aid in the processing of antigen, and promote cel- lular interactions necessary for development of adaptive immune responses. Adaptive immunity can be acquired actively or pas- sively. Active immunity develops through immunization or by having a disease, while passive immunity develops when the host receiving antibodies or immune cells from another source. An acquired immune response can improve with repeated exposure to an injected antigen or a natural infection. After completing this section of the chapter, you should be able to meet the following objectives: •• Explain the transfer of passive immunity from mother to fetus and from mother to infant during breast-feeding. •• Characterize the development of active immunity in the infant and small child. •• Describe the changes in the immune response that occur during the normal aging process. Development of the immune system begins early in fetal life at approximately 5 to 6 weeks gestation when the fetal liver actively begins hematopoiesis.At about the same time (6 weeks gestation), the thymus arises from the third branchial arch with the cortex arising from its ectodermal layer and the medulla from the endoderm. 37,38 Over the next 2 to 3 weeks, lymphoid cells initially migrate from the yolk sac and fetal liver and then from the bone marrow to colonize the fetal thymus. 37,38 Development of the secondary lymphoid organs ( i.e., spleen, lymph nodes, and mucosa-associated lymphoid tissues) begins soon after. The secondary lymphoid organs are rather small but well developed at birth and mature rapidly after exposure to microbes during the postnatal period. The thymus is the largest lymphoid tissue in the neonate relative to body size and nor- mally reaches its mature weight by 1 year of age. Transfer of Immunity from Mother to Infant The neonate’s immune system is functionally immature at birth so protection against infection and toxic substances occurs through transfer of maternal IgG antibodies. Maternal IgG antibodies readily cross the placenta during fetal DEVELOPMENTAL ASPECTS OF THE IMMUNE SYSTEM

IN SUMMARY Adaptive immunity is comprised of two distinct but inter- related processes: cell-mediated and humoral immunity. Together they respond to foreign antigens, amplify and sus- tain immunological responses, distinguish self from non- self, and confer “memory” so that a heightened response can be initiated on subsequent exposure to an organism. Antigens are usually substances foreign to the host that can stimulate an immune response. Antigens possess specific antigenic binding sites for the cells of the immune system known as epitopes. Epitopes allow the adaptive immune system to distinguish foreign antigens from normal cellular substances whose destruction would be detrimental to the organism. The principal cells of the adaptive immune system are the B and T lymphocytes, APCs, and effector cells that are responsible for the elimination of antigens. B lymphocytes differentiate into plasma cells that produce antibodies and provide for the elimination of microbes in the extracellular fluid (humoral immunity) as well as memory cells, which are responsible for the rapid immune response with repeat exposure. T lymphocytes differentiate into regulatory (helper T and regulatory T cells) and effector (cytotoxic T cells) cells. APCs consist of macrophages and DCs that process and present antigen peptides to CD4 + helper T cells. During cellular maturation, T lymphocytes express spe- cific CD molecules on the cellular surfaces that distinguish between the different cell types and that help determine the cells’ functionality. Regulatory CD4 + helper T cells help to modulate the immune response and are essential for the differentiation of B cells into antibody-producing plasma cells and the differentiation of T lymphocytes into effector CD8 + cytotoxic T cells. The CD8 + cytotoxic cells eliminate intracellular microbes, such as viruses and other pathogens. Cells of both the innate and adaptive immune responses pro- duce cytokines that influence adaptive immune responses. These cytokines function as communication molecules for the B and T lymphocytes, stimulate cellular proliferation and differentiation, and ensure the appropriate development of cytotoxic effector and memory cells. Essential to the proper functioning of the adaptive immune response are the cell surface MHC molecules that allow the immune system cells to distinguish self from nonself. Class I MHC complexes that are present are body cells other than those of the immune system, interact with cytotoxic T cells, and present degraded viral protein frag- ments from infected cells for destruction. Class II MHC (MHC-II) complexes are found on immune cells includ- ing phagocytic APCs, immune cells that engulf foreign particles such as macrophages and DCs. Class II MHC complexes also aid in cell-to-cell communication between different cells of the immune system. The cells of the adaptive immune system are present in large numbers in the central and peripheral lymphoid