Porth's Essentials of Pathophysiology, 4e

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Innate and Adaptive Immunity

C h a p t e r 1 5

and proceeds rapidly. Secondary lymphoid organs (i.e., spleen, lymph nodes, and mucosa-associated lymphoid tissues) develop soon after. These 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 fully formed and func- tional and is the largest lymphoid tissue relative to body size at birth.

Developmental Aspects of the Immune System Embryologically, the immune system develops in sev- eral stages, beginning at 5 to 6 weeks as the fetal liver becomes active in hematopoiesis. Development of the primary lymphoid organs (i.e., thymus and bone mar- row) begins during the middle of the first trimester different microbes and pathogens, and remember the specific agents. ■■ Antigens are substances foreign to the host that can stimulate an immune response.They have antigenic determinant sites or epitopes, which the adaptive immune system recognizes with specific receptors that distinguish the antigens as nonself. ■■ The principal cells of the adaptive immune system are theT and B lymphocytes and antigen presenting cells. T lymphocytes differentiate into helperT and regulatoryT cells and cytotoxicT cells and provide cell-mediated immunity. CD4 + helperT cells serve as a trigger for the immune response and are essential for the differentiation of B cells into antibody-producing plasma cells and the differentiation of T lymphocytes into CD8 + cytotoxicT cells. Antigen-presenting cells consist of macrophages and dendritic cells that process and present antigen peptides to CD4 + helperT cells. ■■ Cell surface MHC molecules are key recognition molecules that the immune system uses in distinguishing self from nonself. Class I MHC molecules, which are present on all nucleated cells other than those of the immune system, interact with cytotoxic CD8 + T cells in the destruction of cells that have been affected by intracellular pathogens or cancer. Class II MHC molecules, found on antigen-presenting cells and B lymphocytes, aid in cell-to-cell communication between different cells of the immune system. ■■ Humoral immunity consists of protection provided by the B lymphocyte–derived plasma cells, which produce immunoglobulins that travel in the blood and interact with circulating and cell surface antigens.The immunoglobulins have been divided into five classes, IgG, IgA, IgM, IgD, and IgE, each with a different role in immune defense. ■■ Cell-mediated immunity consists of protection provided by cytotoxicT lymphocytes, which protect against virus-infected or cancer cells.

 Transfer of Immunity from Mother to Infant

Protection of a newborn against antigens occurs through transfer of maternal antibodies. Maternal IgG antibodies cross the placenta during fetal development and remain functional in the newborn for the first few months of life (Fig. 15-14). IgG is the only class of immunoglobulins to cross the placenta. Levels of maternal IgG decrease significantly during the first 3 to 6 months of life, while infant synthesis of immunoglobulins increases. Maternally transmitted IgG is effective against most microorganisms and viruses. The largest amount of IgG crosses the placenta during the last weeks of pregnancy and is stored in fetal tissues; therefore, infants born pre- maturely may be deficient. Because of the transfer of IgG antibodies to the fetus, an infant born to a mother infected with HIV will have a positive HIV antibody test result, although the child may not necessarily be infected with the virus. Cord blood does not normally contain IgM or IgA. If present, these antibodies are of fetal origin and rep- resent exposure to intrauterine infection. The infant begins producing IgM antibodies shortly after birth, in response to the immense antigenic stimulation of his or her new environment. Premature infants appear to be able to produce IgM as well as term infants. At approxi- mately 6 days of age the IgM rises sharply, and this rise continues until approximately 1 year of age, when the adult level is achieved.

1200

1000

800

Total IgG level

600

400 Maternal contribution Serum IgG (mg/dL)

Newborn contribution

200

Fetal contribution

0

1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 8

Months

FIGURE 15-14. Maternal/neonatal serum immunoglobulin G levels. (From J Pediatr. 1968;72(2):276–290.)