Handbook of Immunological Investigations in Children: Handbooks of Investigation in Children

By J. Graham Watson and A. Graham Bird

Handbook of Immunological Investigations in Children presents an extensive examination of the immune system of children. It discusses the fundamental concepts in immunology. It addresses studies in the primary immunodeficiency diseases. Some of the topics covered in the book are the respiratory disorders; renal disorders; gastrointestinal disorders; endocrine disorders; neurological and psychiatric disease; identifications of diseases of the skin; muscle malfunction and connective tissue disorder; and the methods of infections and immunizations. The definition and description of factors influencing blood and blood-forming elements are fully covered. An in-depth account of the factors causing allergies and cardiovascular diseases are provided. The general characteristics of severe combined immunodeficiency are completely presented. A chapter is devoted to usage of immunoglobulin therapy. Another section focuses on the disorders of neutrophils, monocytes, and macrophages. The book can provide useful information to immunologists, pediatricians, doctors, students, and researchers.

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 28, 2014
• ISBN: 9781483183657

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Chapter 1

Basic immunology

Publisher Summary

This chapter provides an overview of basic immunology. The immune response has evolved to provide a comprehensive system of defense against microbiological attack. Immune responses can be divided into two major functional compartments, both of which have recognition provided by the lymphocyte population and its products. These are linked to activation of the effector pathways responsible for antigen or microorganism removal. Antigen-specific cellular immunity is generated by specific T lymphocytes and often also involves the recruitment and activation of nonspecific effector cells. In cellular immune responses the effector cells are the monocyte-macrophage and the recruitment factors are the lyphokines that are released from activated specific T cells. Antigen presenting cells (macrophages) fulfill a role of protein digestion and presentation in an appropriate form for helper T cells. Whereas, antibody can recognize antigens present in whole bacteria, virus, or parasites, the cytotoxic T cell is incapable of this, but will identify changes in the cell membrane or major histocompatibility complex antigens of infected cells.

This chapter does not attempt to replace the excellent texts available that provide detailed description of the organization and generation of immune responses. The intention of this introduction is merely to provide a simple background to the practical knowledge necessary to allow the reader to use the subsequent text.

Functional organization of the immune response

The immune response has evolved to provide a comprehensive system of defence against microbiological attack. In vertebrates, the system of specific recognition of antigens provided by lymphocyte responses has been added to the non-specific phagocytic and serum effector mechanisms present in invertebrates. A key feature of lymphocyte-based responses is that of memory which allows the previous experience of the individual to produce rapid, more effective subsequent protection and also allows the transfer of temporary protection to an offspring. Another key feature is that antibody and T-cell responses can be predictive. The random generation of specificities that result from DNA rearrangement as lymphocytes develop results in a broad repertoire of processes for antigen recognition which is not based on previous experience of the species but is capable of predicting and reacting to the emergence of new bacterial or viral species and thus ensuring the survival of a proportion of each generation.

Humoral and cellular immunity

Immune responses can be divided into two major functional compartments, both of which have recognition provided by the lymphocyte population and its products. These are linked to activation of the effector pathways responsible for antigen or micro-organism removal.

Humoral immunity

Antibody responses are generated by B lymphocytes and result in the appearance of immunoglobulin classes each of which bears identical antibody-combining sites. Each class, however, has different effector functions. Antibody responses to protein antigens require close interaction with antigen presenting cells and T helper cells to produce specific responses and good immunological memory. Polysaccharide antigens (important constituents of many bacterial cell walls) can elicit antibody responses in the absence of T cells, but there is little generation of memory. A major way in which antibody produces its effect is via the complement system, which enhances the antibody recognition of antigen, creates additional opsonization, thereby enhancing phagocytosis, and also recruits polymorphs to sites of antibody–antigen reaction. The neutrophil polymorph is the major effector cell of the antibody response and is the characteristic cell seen histopathologically in antibody-based inflammation.

The major functions and properties of individual immunoglobulin classes are given in Table 1.1.

Table 1.1

Immunoglobulin classes and functions

¹PMN: polymorph

IgE should be regarded as a specialized immunoglobulin class all on its own, producing protection and inflammation not by complement activation but by mast-cell sensitization and eosinophil recruitment. Phylogenetically the IgE system plays a major role in parasite defence, but in developed nations IgE responses are now most commonly encountered in hypersensitivity reactions in atopic subjects and drug allergic states. IgA is the major mucosal immunoglobulin, coating mucosal surfaces and protecting against bacterial adhesion or viral penetration. IgA has two subclasses IgA1 and IgA2. IgA1 dominates in the serum but IgA2 is equally represented with IgA1 in secretions including human milk and is preferentially resistant to bacterial protease attack and acid digestion.

IgM forms the early and transient response to neoantigens. IgG possesses a long half-life and low molecular weight, giving it properties of long-lasting protection and good tissue penetration. The vertical transmission from mother to offspring of passive IgG gives important protection to the neonate as the developing immune response matures. IgG is also used in immunoglobulin therapy to allow long- and short-term passive protection in immunodeficient or immunologically naïve individuals.

Antibody responses against most natural infections are polyclonal and comprise individual responses from a number of parent B-cell clones with overlapping specificities. Monoclonal antibody responses indicate origin from a single B-cell clone and generally characterize adult B lymphocyte malignancy. Each B cell has a determined specificity and the ability to activate and immortalize these individual B cells by cell fusion has allowed the production of unlimited quantities of monoclonal antibody of defined and predictable specificity. Such preparations are being increasingly used in diagnostic techniques and have an emerging use in clinical therapy.

Cellular immunity

Antigen-specific cellular immunity is generated by specific T lymphocytes and often also involves the recruitment and activation of non-specific effector cells. In cellular immune responses the effector cells is the monocyte–macrophage and the recruitment factors are the lymphokines (now renamed cytokines) that are released from activated specific T cells. A crucial difference between the T cell and the B cell is in the nature of the antigen which it ‘recognizes’. Whereas the B cell and its secreted product, antibody, recognizes whole and extracellular antigens, the T lymphocyte is incapable of such recognition and can only ‘see’ small linear peptides of digested protein presented as part of a cell membrane and in direct association with the membrane major histocompatibility (MHC) antigens. Antigen presenting cells (macrophages) fulfil a role of protein digestion and presentation in an appropriate form for ‘helper’ T cells. Whereas antibody can recognize antigens present in whole bacteria, virus, or parasites, the cytotoxic T cell is incapable of this, but will identify changes in the cell membrane or MHC antigens of infected cells.

The T-cell receptor (for antigen together with MHC proteins) is a two-chain structure and like immunoglobulin can display a range of specificities due to rearrangement of the DNA in developing T cells. The T-cell receptor recognizes processed peptide antigens (T cells cannot see polysaccharides) in association with class I (HLA, A, B) or class II (HLA DR) MHC antigens. Class I antigens are variably expressed on most nucleated cells whereas class II antigens are expressed on a limited number of cells most of which are macrophage or B lymphocyte cell types that can process and present antigen.

Antigen presentation consists of the internalization of antigen by pinocytosis, followed by degradation and re-expression of the peptides in close association with class II MHC molecules. Such peptides are recognized by class II restricted lymphocytes bearing T cell receptors, i.e. the response of these lymphocytes is restricted to when they are stimulated by antigen borne by cells bearing class II (HLA DR) markers. The vast majority of these class II restricted lymphocytes possess helper function and express the CD4 antigen on their membrane. This population of T cells is responsible for release of cytokines, the three most important of which are:

The other major population of T cells expresses CD8 antigen on their membrane, and is a class I (HLA A, B) restricted population. These cells are not important producers of cytokines, and mediate destruction of virus-infected cells by cytotoxic killing. This population also contains cells capable of suppressing immune responses. Virus-infected cells process peptides derived from the infecting agent and expose these along with class I MHC antigens on their cell membrane. Such cells can be identified as infected and then killed by cytotoxic antigen-specific CD8-positive lymphocytes.

These prerequisites determine that the T lymphocyte is an important defenct against intracellular infection; a function which antibodies are ill-suited to perform.

The practical dichotomy of antibody and cellular responses in clinical immunology

In microbiological defence, antibody-based recognition is of preeminent importance in the primary defence against bacterial infections. As already indicated, T cells are incapable of recognizing bacterial polysaccharide antigens and cannot identify or attack extracellular infection. Thus, antibody deficiency presents with a specific vulnerability to repeated or severe episodes of bacterial infection. Although antibody if pre-existing (as in the neonate) is effective in neutralizing and therefore preventing primary virus infection, once intracellular virus infection is established, only T-cell mechanisms can limit virus replication and eliminate infection or maintain latency. Thus, selective antibody deficiency is not characterized by increased severity of viral infection, which can be dealt with effectively by T-cell mechanisms alone.

In contrast, T-cell immunity is the essential protection against intracellular infection or virus-induced tumours. In certain infections, e.g. mycobacterial infection, immunity comprises the limitation of infection to within macrophages in which the organism can replicate. The characteristic lesion, the granuloma, results from cytokine recruitment of effector macrophages by specific CD4 cells. Such immunopathology also characterizes the delayed type hypersensitivity (Gell and Coombes type IV) reaction typified by the reaction to an intradermal challenge with tuberculin. In virus infection, destruction by class I restricted CD8 cells is the means of protection. Since CD8 cells require interleukin 2 produced by CD4 cells for their clonal proliferation, depletion of the CD4 subpopulation which characterizes HIV infection results in the comprehensive impairment of immune response against the intracellular infections that characterize the acquired immunodeficiency syndrome.

The strictures imposed by T-cell recognition also limit the ability to investigate function in the immunology laboratory. A requirement of class I identification makes it practically very difficult to assess CD8 cell function in man. CD4 function can be examined more simply by measuring the T-cell proliferation to antigens or mitogens by CD4 cells or by interleukin 2 release. The migration inhibition test of neutrophils or monocytes is also a useful in-vitro test of delayed type hypersensitivity and cytokine release and does not require HLA matching because it uses autologous cells. Cytotoxic responses need HLA-matched target cells for analysis. This has severely compromised scientific approaches in man and thus the role of T-cell immunity in many autoimmune diseases awaits the identification of autoimmunizing antigens and satisfactory assay systems.

In contrast, antibodies are (relatively) freely exchanged from one individual to another without loss of function, and require no HLA matching to identify responses. The ease of analysis of antibody responses has almost certainly led to exaggeration of their importance as initiators or perpetrators of some autoimmune diseases, and in many of these diseases the role of T-cell immunity (a much more realistic immunological explanation) remains to be examined. Molecular biology and our ability to clonally expand individual T-cell populations make this next phase of immunological investigation a realistic possibility. This will mean that many of the frustrating uncertainties in sections of this volume will probably not remain so for much longer.

Chapter 2

Primary immunodeficiency diseases

Publisher Summary

This chapter reviews the major primary immunodeficiency syndromes, their presentation, and the most appropriate investigations to identify a deficiency and its associated features. Many syndromes do not fall neatly into a classification based on a single affected cell type because, with greater understanding, few immunodeficiencies involve only one cell type or a single differentiation or maturation pathway. The chapter describes severe combined immunodeficiency, which is a profound heterogeneous deficiency of both cell-mediated and antibody immunity. Such deficiency is usually congenital, but a similar immunological state may arise secondary to thymoma, cancer therapy, or retro virus infection. Autosomal recessive, sex-linked, and sporadic forms result in four affected males to each female. The chapter also describes Wiskott–Aldrich syndrome, which is a rare X-linked immunodeficiency. Ataxia–Telangiectasia is an autosomal recessive immunodeficiency usually present in early childhood with progressive cerebellar ataxia or recurrent respiratory tract infection.

Introduction

This section reviews the major primary immunodeficiency syndromes, their presentation, and the most appropriate investigations to identify a deficiency and its associated features. Many syndromes do not fall neatly into a classification based on a single affected cell type because, with greater understanding, few immunodeficiencies involve only one cell type or a single differentiation or maturation pathway. Conditions are grouped according to the most clinically important deficiency. The basic investigative techniques and the normal values for the results are in Chapter 15.

The incidence of primary immunodeficiency in the United Kingdom is not known. The MRC 1969 report suggested 1 case per 100000 population, which is now regarded as a considerable underestimate. Estimates from the USA and Australia suggest a minimum annual incidence of 1 in 10000 births, with an incidence for individual conditions of 1 in 60000 for combined immunodeficiency and DiGeorge syndrome; 1 in 80000 for common variable immunodeficiency in children; 1 in 100000 for chronic mucocutaneous candidiasis and X-linked hypogammaglobulinemia; and 1 in 180000 for chronic granulomatous disease (Stiehm, 1989). Alternatively it has been suggested that about 50–100 children with severe immunodeficiency warranting bone-marrow transplantation are born each year in the United Kingdom (Hobbs, 1981). These severe diseases, however, are only a few of the conditions either contributing to or caused by immunodeficiency. The clinical demand derived from these data can be considerably altered locally by the prevalence of cousin marriages, prenatal diagnosis, and termination of