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Though nonspecific host defenses are somewhat deficient in the infant and pediatric patient, the effector arm of the specific immunologic response to alloantigens of clinical organ transplants is intact at the time of birth.
Effective organ allograft immunosuppression requires a balance of prevention of rejection of the graft while avoiding the toxicity of excessive immunosuppressing agents.
Immunosuppressive agents can be categorized into those used for induction (rabbit anti-thymocyte globulin, corticosteroids), those for maintenance (mycophenolate mofetil, tacrolimus), and those used to treat acute rejection (corticosteroids).
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Infections are a leading cause of pediatric morbidity and mortality, particularly in the neonatal period. Explanations for this phenomenon include a lack of previous antigenic experience on the part of pediatric patients; an intrinsic immaturity of lymphocyte functions in this age group; and, for neonates, an active cellular suppression mechanism. In addition, several specific inherited defects of the immune system may manifest themselves in early infancy and continue to plague these patients through childhood and on into adulthood. Acquired defects in the immune defenses occur in transplantation and oncology patients. This chapter will review the specific and nonspecific components of the immune system with particular reference to the pediatric patient's ability to fight infection (see also Chapter 13—“Surgical Infection: Classification, Diagnosis, Treatment and Prevention”). Emphasis will be placed on the differences in these responses in neonates and children compared to the responses in adult subjects. The ability of children to mount an immune response against alloantigens in the transplantation setting will be discussed. In addition, this chapter will review current methods of clinical immunosuppression for the pediatric age group.
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Components of the Immune System
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Immune responses are generally divided into specific and nonspecific components. Each of these components is further subdivided into humoral and cellular compartments (Table 97-1).
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Nonspecific (Nonimmune) Host Defenses
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The humoral components of nonspecific host defense systems include the classic complement pathway and the alternate, or properdin, pathway. The complement cascade is made up of 9 serum proteins which are activated during the inflammatory response. The proteins react with immunoglobulins (Ig)—IgM or IgG—and antigen. Activated complement (C′) liberates mediators of the inflammatory response, and the levels of the C3/C5 component correlate with susceptibility to infection. The C3 level determines the rate of phagocytosis or lysis of invading organisms. In the presence of immunoglobulins, the C142 component (the membrane attack complex) activates C3, the binding ...