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• W2084343696 abstract "Immunoglobulin G—which can be subdivided into four classes, each with different functional characteristics—is an important component of the host defense system of the respiratory tract. An excessive amount can be produced or can accumulate after airway irritation (exposure to cigarette smoke) or from immunologic stimulus of B-lymphocyte-plasma cells in types of hypersensitivity and interstitial lung diseases. Specific antibody activity can be identified in organic dust-induced hypersensitivity pneumonitis and asthma that contributes to disease pathogenesis. The availability of opsonic antimicrobial antibodies is essential for optimal function of phagocytes in uptake and containment of bacteria. With an absolute or functional deficiency of IgG, recurrent and chronic types of sinopulmonary infections occur. These extremes of IgG availability, either high levels (presumably excessive) or deficient, are discussed in this review. Immunoglobulin G—which can be subdivided into four classes, each with different functional characteristics—is an important component of the host defense system of the respiratory tract. An excessive amount can be produced or can accumulate after airway irritation (exposure to cigarette smoke) or from immunologic stimulus of B-lymphocyte-plasma cells in types of hypersensitivity and interstitial lung diseases. Specific antibody activity can be identified in organic dust-induced hypersensitivity pneumonitis and asthma that contributes to disease pathogenesis. The availability of opsonic antimicrobial antibodies is essential for optimal function of phagocytes in uptake and containment of bacteria. With an absolute or functional deficiency of IgG, recurrent and chronic types of sinopulmonary infections occur. These extremes of IgG availability, either high levels (presumably excessive) or deficient, are discussed in this review. Among the components of humoral immunity, immunoglobulin G (IgG) is prominent as the repository for an important class of antibodies that can function as opsonins or agglutinins or can interact with the complement cascades. Over the mucosa of the respiratory tract and the alveolar epithelial surface, IgG is also present in the lining fluid and secretions. It functions in concert with IgA as part of the local airway host defense against microbial agents, organic antigens, or similar substances that can be aspirated or aerosolized into the airways and must be contained or eliminated.1Reynolds HY Chrétien J Respiratory tract fluids: analysis of content and contemporary use in understanding lung diseases.DM. February 1984; 30: 1-103PubMed Google Scholar Most of the IgG normally reaches the airways by transudation from plasma, but it can be added locally or directly in the airways as well; thus, local mechanisms may regulate its synthesis also. Deficiencies of IgG are part of a systemic abnormality in humoral immunity and are not limited to the lungs; however, these disorders may disproportionately involve this anatomic area. When allergic or hypersensitivity diseases are present, excessive amounts of IgG are often found in secretions—frequently reflected as increased amounts of specific antibody to an identified antigen. For lung diseases of still unknown cause, an antibody response presumably may have occurred but the specific reactivity cannot be identified. Alternatively, such a disease may reflect disordered immunoregulation, and the excessive immunoglobulin might be a nonspecific response or may cause nonspecific lung injury through immune complex formation. As IgG is a composite of four distinct subclasses, individual antibodies identified among the respective subclasses provide a clue about the complexity of the host's response. Table 1 outlines some of their individual characteristics.2Grey HM Kunkel HG H chain subgroups of myeloma proteins and normal 7S γ-globulin.J Exp Med. 1964; 120: 253-266Crossref PubMed Scopus (173) Google Scholar, 3Terry WD Fahey JL Subclasses of human γ2-globulin based on differences in the heavy polypeptide chains.Science. 1964; 146: 400-401Crossref PubMed Scopus (118) Google Scholar, 4Frangione B Milstein C Pink JRL Structural studies of immunoglobulin G.Nature. 1969; 221: 145-148Crossref PubMed Scopus (106) Google Scholar, 5Morell A Terry WD Waldmann TA Metabolic properties of IgG subclasses in man.J Clin Invest. 1970; 49: 673-680Crossref PubMed Scopus (382) Google Scholar, 6Schur PH Human gamma-G subclasses.Prog Clin Immunol. 1972; 1: 71-104PubMed Google Scholar, 7Kronvall G Williams Jr, RC Differences in anti-protein A activity among IgG subgroups.J Immunol. 1969; 103: 828-833PubMed Google Scholar, 8Ochs HD Wedgwood RJ IgG subclass deficiencies.Annu Rev Med. 1987; 38: 325-340Crossref PubMed Scopus (50) Google Scholar, 9Naegel GP Young Jr, KR Reynolds HY Receptors for human IgG subclasses on human alveolar macrophages.Am Rev Respir Dis. 1984; 129: 413-418PubMed Google Scholar, 10Shapiro SS Characterization of factor VIII antibodies.Ann NY Acad Sci. 1975; 240: 350-360Crossref PubMed Scopus (23) Google Scholar, 11Siber GR Schur PH Aisenberg AC Weitzman SA Schiffman G Correlation between serum IgG-2 concentrations and the antibody response to bacterial polysaccharide antigens.N Engl J Med. 1980; 303: 178-182Crossref PubMed Scopus (340) Google Scholar, 12Umetsu DT Ambrosino DM Quinti I Siber GR Geha RS Recurrent sinopulmonary infection and impaired antibody response to bacterial capsular polysaccharide antigen in children with selective IgG-subclass deficiency.N Engl J Med. 1985; 313: 1247-1251Crossref PubMed Scopus (280) Google Scholar, 13Fick Jr, RB Olchowski J Squier SU Merrill WW Reynolds HY Immunoglobulin-G subclasses in cystic fibrosis: IgG2 response to Pseudomonas aeruginosa lipopolysaccharide.Am Rev Respir Dis. 1986; 133: 418-422PubMed Google Scholar, 14Sundqvist V-A Linde A Kurth R Werner A Helm EB Popovic M Gallo RC Wahren B Restricted IgG subclass responses to HTLV-III/LAV and to cytomegalovirus in patients with AIDS and lymphadenopathy syndrome.J Infect Dis. 1986; 153: 970-973Crossref PubMed Scopus (52) Google Scholar Emphasis on the subclasses underscores their unique activities because absence of certain ones can predispose the host to sinopulmonary infections.Table 1Structural and Functional Characteristics of Human IgG Subclasses*Data compiled from references 2–14.Enzyme digestion†R = resistant; S = susceptible.IgG subclassMolecular weight (x103)Heavy chain molecular weight (x103)Inter-heavy chain disulfide bondsBiologic half-life (days)Mean values in normal serum (mg/ml)Fixation of complementReceptors on alveolar macrophagesPapainPepsinRestricted antibody toward‡AIDS = acquired immunodeficiency syndrome; CMV = cytomegalovirus; HIV = human immunodeficiency virus; ITP = idiopathic thrombocytopenic purpura.IgG114651.6221.2 ± 5.46.7–10.5YesYesSRA isoagglutinins, Rh, diphtheria or tetanus toxoid, viruses (HIV, CMV in AIDS)IgG214651.6420.2 ± 1.92.5–4.2VariableNoR±RTetanus toxoid, teichoic acid, polysaccharides, lipopoly-saccharidesIgG316559.5§Hinge region of IgG3 is 47 amino acid residues longer than that of IgG1.117.1 ± 0.70.54–1.0YesYes (many)SSRh, viral neutralizing, platelets (in ITP), HIV, CMVIgG414651.6220.6 ± 2.60.36–0.67NoNoRSCirculating anticoagulants, factors VIII and IX, phospholipase A (chronic stimulus)* Data compiled from references 2Grey HM Kunkel HG H chain subgroups of myeloma proteins and normal 7S γ-globulin.J Exp Med. 1964; 120: 253-266Crossref PubMed Scopus (173) Google Scholar, 3Terry WD Fahey JL Subclasses of human γ2-globulin based on differences in the heavy polypeptide chains.Science. 1964; 146: 400-401Crossref PubMed Scopus (118) Google Scholar, 4Frangione B Milstein C Pink JRL Structural studies of immunoglobulin G.Nature. 1969; 221: 145-148Crossref PubMed Scopus (106) Google Scholar, 5Morell A Terry WD Waldmann TA Metabolic properties of IgG subclasses in man.J Clin Invest. 1970; 49: 673-680Crossref PubMed Scopus (382) Google Scholar, 6Schur PH Human gamma-G subclasses.Prog Clin Immunol. 1972; 1: 71-104PubMed Google Scholar, 7Kronvall G Williams Jr, RC Differences in anti-protein A activity among IgG subgroups.J Immunol. 1969; 103: 828-833PubMed Google Scholar, 8Ochs HD Wedgwood RJ IgG subclass deficiencies.Annu Rev Med. 1987; 38: 325-340Crossref PubMed Scopus (50) Google Scholar, 9Naegel GP Young Jr, KR Reynolds HY Receptors for human IgG subclasses on human alveolar macrophages.Am Rev Respir Dis. 1984; 129: 413-418PubMed Google Scholar, 10Shapiro SS Characterization of factor VIII antibodies.Ann NY Acad Sci. 1975; 240: 350-360Crossref PubMed Scopus (23) Google Scholar, 11Siber GR Schur PH Aisenberg AC Weitzman SA Schiffman G Correlation between serum IgG-2 concentrations and the antibody response to bacterial polysaccharide antigens.N Engl J Med. 1980; 303: 178-182Crossref PubMed Scopus (340) Google Scholar, 12Umetsu DT Ambrosino DM Quinti I Siber GR Geha RS Recurrent sinopulmonary infection and impaired antibody response to bacterial capsular polysaccharide antigen in children with selective IgG-subclass deficiency.N Engl J Med. 1985; 313: 1247-1251Crossref PubMed Scopus (280) Google Scholar, 13Fick Jr, RB Olchowski J Squier SU Merrill WW Reynolds HY Immunoglobulin-G subclasses in cystic fibrosis: IgG2 response to Pseudomonas aeruginosa lipopolysaccharide.Am Rev Respir Dis. 1986; 133: 418-422PubMed Google Scholar, 14Sundqvist V-A Linde A Kurth R Werner A Helm EB Popovic M Gallo RC Wahren B Restricted IgG subclass responses to HTLV-III/LAV and to cytomegalovirus in patients with AIDS and lymphadenopathy syndrome.J Infect Dis. 1986; 153: 970-973Crossref PubMed Scopus (52) Google Scholar.† R = resistant; S = susceptible.‡ AIDS = acquired immunodeficiency syndrome; CMV = cytomegalovirus; HIV = human immunodeficiency virus; ITP = idiopathic thrombocytopenic purpura.§ Hinge region of IgG3 is 47 amino acid residues longer than that of IgG1. Open table in a new tab In this review, values for IgG (especially the subclasses) in the normal respiratory tract will be presented first, followed by examples of increased or excessive amounts of IgG that can be found in normal cigarette smokers and in patients with certain types of respiratory disease. Finally, the influence of deficient amounts of IgG will be examined, as this may relate to recurrent infections. IgG is present in mucosal secretions in the nose and along the tracheobronchial tree, but it is in small amounts relative to the concentrations of total protein and of IgA.15Reynolds HY Lung immunology and its contribution to the immunopathogenesis of certain respiratory diseases.J Allergy Clin Immunol. 1986; 78: 833-847Abstract Full Text PDF PubMed Scopus (6) Google Scholar Mucosal fluids from the trachea and the largest intrapulmonary-conducting airways are the least precisely characterized of the fluids from the human respiratory tract because available sampling techniques do not limit or localize the specimen well to this particular mucosal area. In early studies of bronchial lavage, very small quantities of saline were used to recover secretions from patients without known lung disease who were intubated for a surgical procedure or were undergoing rigid bronchoscopy. These small “washings” probably sampled the larger airways preferentially instead of the alveolar surface. In three reports,16Keimowitz RI Immunoglobulins in normal human tracheobronchial washings: a qualitative and quantitative study.J Lab Clin Med. 1964; 63: 54-59PubMed Google Scholar, 17Masson PL Heremans JF Prignot J Studies on the proteins of human bronchial secretions.Biochim Biophys Acta. 1965; 111: 466-478Crossref PubMed Scopus (33) Google Scholar, 18Falk GA Okinaka AJ Siskind GW Immunoglobulins in the bronchial washings of patients with chronic obstructive pulmonary disease.Am Rev Respir Dis. 1972; 105: 14-21PubMed Google Scholar IgG was present in bronchial secretions in approximately the same proportion to albumin as is found in serum. In contrast, IgA was increased severalfold relative to albumin and, therefore, considered to be in part locally secreted into the airways. A direct comparison of the amount of IgG to IgA in nasal washings and bronchoalveolar lavage (BAL) fluid revealed ratios of about 1:3 and 2.5:1, respectively;19Waldman RH Jurgensen PF Olsen GN Ganguly R Johnson III, JE Immune response of the human respiratory tract. I. Immunoglobulin levels and influenza virus vaccine antibody response.J Immunol. 1973; 111: 38-41PubMed Google Scholar thus, IgG is relatively increased in the lower respiratory tract. For washing portions of the lung to recover airway-alveolar surface secretions, the fiberoptic bronchoscope is currently preferred, and most specimens of BAL fluid recently analyzed have been obtained with this instrument. The details of BAL and methods used to analyze cells and proteins in BAL fluid have been thoroughly reviewed previously.1Reynolds HY Chrétien J Respiratory tract fluids: analysis of content and contemporary use in understanding lung diseases.DM. February 1984; 30: 1-103PubMed Google Scholar, 20Reynolds HY Bronchoalveolar lavage.Am Rev Respir Dis. 1987; 135: 250-263PubMed Google Scholar Normal subjects, usually between 19 and 30 years of age, have undergone BAL in one or several subsegments of a lung with 100 to 300 ml of saline. Approximately 60% of the lavage fluid is recovered from normal subjects, and the total BAL specimen may contain about 15 mg of protein.21Merrill WW Goodenberg D Strober W Matthay RA Naegel GP Reynolds HY Free secretory component and other proteins in human lung lavage.Am Rev Respir Dis. 1980; 122: 156-161PubMed Google Scholar Albumin constitutes approximately a third of this total and accounts for the largest share of measurable protein in airway lavage fluid. In contrast, albumin accounts for approximately 60% of the total protein in normal serum. Representative values of IgG in BAL fluid based on a ratio with albumin are shown in Table 2, as are values in serum from nonsmokers and cigarette smokers. For nonsmokers, the relative proportion of IgG in BAL fluid is approximately the same as in serum. Although IgG (molecular weight about 150,000 daltons) and albumin (about 68,000 daltons) differ considerably in size, they diffuse through the blood-alveolar membrane interface in roughly similar amounts. Smokers seem to have a disproportionately greater amount of IgG in BAL fluid than in serum.Table 2IgG Values*Data are presented as mean values; ND = not determined. in Bronchoalveolar Lavage (BAL) Fluid and Serum From Normal Nonsmokers and Cigarette Smokers Described in the LiteratureNonsmokersSmokersBAL fluidSerumBAL fluidSerumReferenceNVolume (ml)% returnTotal protein (mg/ml)IgG/albuminIgG/albuminNVolume (ml)% returnTotal protein (mg/ml)IgG/albuminIgG/albuminReynolds & Newball22Reynolds HY Newball HH Analysis of proteins and respiratory cells obtained from human lungs by bronchial lavage.J Lab Clin Med. 1974; 84: 559-573PubMed Google Scholar5150644.20.120.235150745.60.320.21Warr et al23Warr GA Martin RR Sharp PM Rossen RD Normal human bronchial immunoglobulins and proteins: effects of cigarette smoking.Am Rev Respir Dis. 1977; 116: 25-30PubMed Google Scholar36250586.17 mg0.340.2519250596.94 mg0.5250.25Low et al24Low RB Davis GS Giancola MS Biochemical analyses of bronchoalveolar lavage fluids of healthy human volunteer smokers and nonsmokers.Am Rev Respir Dis. 1978; 118: 863-875PubMed Google Scholar142407367.60.20†Seven samples tested for nonsmokers and for smokers.0.18122406984.50.26†Seven samples tested for nonsmokers and for smokers.0.20Merrill et al21Merrill WW Goodenberg D Strober W Matthay RA Naegel GP Reynolds HY Free secretory component and other proteins in human lung lavage.Am Rev Respir Dis. 1980; 122: 156-161PubMed Google Scholar242006217 mg0.260.30322006114 mg0.400.30Bell et al25Bell DY Haseman JA Spock A McLennan G Hook GER Plasma proteins of the bronchoalveolar surface of the lungs of smokers and nonsmokers.Am Rev Respir Dis. 1981; 124: 72-79PubMed Google Scholar12150680.160.28‡Ratio calculated from mean data reported in article.0.2411150620.280.460.23Merrill et al26Merrill W O'Hearn E Rankin J Naegel G Matthay RA Reynolds HY Kinetic analysis of respiratory tract proteins recovered during a sequential lavage protocol.Am Rev Respir Dis. 1982; 126: 617-620PubMed Google Scholar14§Subjects included 8 smokers and 6 nonsmokers, but lavage results were the same and therefore pooled.30055418 μg0.24ND* Data are presented as mean values; ND = not determined.† Seven samples tested for nonsmokers and for smokers.‡ Ratio calculated from mean data reported in article.§ Subjects included 8 smokers and 6 nonsmokers, but lavage results were the same and therefore pooled. Open table in a new tab As the respective subclasses of IgG account for many of the distinctive functions of IgG antibodies, quantitation of the subclasses in respiratory fluids is important. In 1966, Rossen and colleagues27Rossen RD Butler WT Vannier WE Douglas Jr, RG Steinberg AG The sedimentation and antigenic properties of proteins in nasal and other external secretions.J Immunol. 1966; 97: 925-938PubMed Google Scholar identified IgG subclasses qualitatively in nasal wash secretions. On immunoelectrophoresis and agar gel precipitin reactions, they found γGb in all eight specimens, γGa and γGc in four and five specimens, respectively, but no γGd. Subsequently, the nomenclature for immunoglobulins was changed to designate the subclasses as IgG1, IgG2, IgG3, and IgG4. IgG in BAL fluid is composed of the four heavy-chain subclasses, and these are quantitatively in approximately the same proportions as found in serum.6Schur PH Human gamma-G subclasses.Prog Clin Immunol. 1972; 1: 71-104PubMed Google Scholar, 28Merrill WW Naegel GP Olchowski JJ Reynolds HY Immunoglobulin G subclass proteins in serum and lavage fluid of normal subjects: quantitation and comparison with immunoglobulins A and E.Am Rev Respir Dis. 1985; 131: 584-587PubMed Google Scholar When smokers were compared with nonsmokers, serum levels of IgG1 were found to be higher in smokers; however, serum levels of other subclasses were similar in these two groups. Analysis of lavage fluids revealed that relative levels of IgG1 and IgG2 were similar to levels found in serum and showed a good correlation (r = 0.57, P = 0.02; r = 0.58, P = 0.02, respectively). As in serum, mean lavage levels of IgG1 for smokers exceeded the mean IgG1 for nonsmokers; however, the relative serum and lavage levels of IgG1 in smokers were similar. In Table 3, IgG subclass values in serum and BAL fluid are compared but have been computed for each subclass as the relative percentage of total IgG present in the specimen.Table 3Quantification of IgG Subclasses*As percentage of total IgG, presented as means ± SEM. in Serum and Bronchoalveolar Lavage (BAL) Fluid of Normal Nonsmokers and Cigarette SmokersModified from Merrill and associates.28Merrill WW Naegel GP Olchowski JJ Reynolds HY Immunoglobulin G subclass proteins in serum and lavage fluid of normal subjects: quantitation and comparison with immunoglobulins A and E.Am Rev Respir Dis. 1985; 131: 584-587PubMed Google ScholarNonsmokers (N = 19)Smokers (N = 12)SubclassSerumBAL fluidSerumBAL fluidIgG167.0 ± 2.465.0 ± 2.480.0 ± 2.479.0 ± 2.0IgG231.0 ± 0.128.0 ± 0.918.0 ± 0.613.0 ± 0.5IgG30.4 ± 0.011.8 ± 0.20.5 ± 0.23.7 ± 0.3IgG41.3 ± 0.025.2 ± 0.61.0 ± 0.034.6 ± 0.1* As percentage of total IgG, presented as means ± SEM. Open table in a new tab Assessment of lavage levels of IgG3 and IgG4 revealed an interesting contrast. The relative levels of IgG4 in lavage fluid were substantially increased in comparison with those in serum for both smokers and nonsmokers. In smokers, the relative level of IgG3 in lavage fluid exceeded the serum level. Finally, although serum levels of all IgG subclasses and lavage levels of IgG1 and IgG2 were normally distributed, lavage levels of IgG3 and IgG4 demonstrated nongaussian distribution (nonparametrically distributed) in both smokers and nonsmokers. Overall, IgG subclass proteins display heterogeneity with respect to their relative local concentration in the lung. IgG1 and IgG2 seem to depend primarily on transudation from serum for their presence in lung lining fluid. IgG4 (and IgG3 in some subjects) is increased relative to its serum concentration. The mechanism for this increase is uncertain although potentially important factors include increased local synthesis and accumulation. At least three mechanisms may contribute to the concentration of IgG found in airway and alveolar space secretions: (1) transudation or diffusion from plasma across the blood-air interface, (2) local secretion from the airway mucosa and by intraluminal lymphocytes, and (3) differential accumulation in secretions, which may result from variable synthesis or catabolism of IgG subclasses.29Reynolds HY Merrill WW Pulmonary immunology: humoral and cellular immune responsiveness of the respiratory tract.Curr Pulmonology. 1981; 3: 381-422Google Scholar Diffusion of plasma IgG into respiratory secretions of the normal, nonirritated, noninflamed lung probably occurs readily and is the major pathway for entry of this class of immunoglobulins. As reviewed in Table 2, the relative amounts of IgG and albumin are maintained in serum and in BAL fluid despite a considerable difference in molecular size of the respective proteins. The selective permeability of the capillary endothelial cell layer-interstitial space-alveolar type I cell epithelial layer (or, in aggregate, the alveolar-capillary interface) allows globular proteins of the IgG size and configuration to pass readily. Little is known about local transport mechanisms through these cell types (pinocytosis) or whether transport may occur at intercellular junctions. Moreover, what fractional percentage of the serum concentration of IgG is cleared into the lungs is unknown. In animal experiments that have used homologous IgG,30Reynolds HY Thompson RE Pulmonary host defenses. I. Analysis of protein and lipids in bronchial secretions and antibody responses after vaccination with Pseudomonas aeruginosa.J Immunol. 1973; 111: 358-368PubMed Google Scholar, 31Kazmierowski JA Durbin WA Reynolds HY Kinetics of immunoglobulin transport into canine bronchial secretions.Proc Soc Exp Biol Med. 1976; 152: 493-498Crossref PubMed Scopus (9) Google Scholar the amount of circulating immunoglobulin that diffuses into the secretions of the lower respiratory tract is small but proportional to serum or plasma levels, a finding that suggests that a concentration gradient favors diffusion. Similarly, for two of the IgG subclass values in humans—IgG1 and IgG2—serum values correlated with respective BAL fluid values.28Merrill WW Naegel GP Olchowski JJ Reynolds HY Immunoglobulin G subclass proteins in serum and lavage fluid of normal subjects: quantitation and comparison with immunoglobulins A and E.Am Rev Respir Dis. 1985; 131: 584-587PubMed Google Scholar IgG can be directly added into respiratory secretions by its synthesis in plasma cells or lymphocytes located in the submucosa of the respiratory tree32Soutar CA Distribution of plasma cells and other cells containing immunoglobulin in the respiratory tract of normal man and class of immunoglobulin contained therein.Thorax. 1976; 31: 158-166Crossref PubMed Scopus (35) Google Scholar and by intraluminal secretion, or release, by lymphocytes that have been characterized in BAL fluids of humans.33Rankin JA Naegel GP Schrader CE Matthay RA Reynolds HY Air-space immunoglobulin production and levels in bronchoalveolar lavage fluid of normal subjects and patients with sarcoidosis.Am Rev Respir Dis. 1983; 127: 442-448Crossref PubMed Scopus (42) Google Scholar In one study of normal nonsmokers, 1,100 ± 477 IgG-releasing lymphocytes per 106Schur PH Human gamma-G subclasses.Prog Clin Immunol. 1972; 1: 71-104PubMed Google Scholar lymphocytes in BAL fluid were detected with use of a reverse hemolytic plaque assay.33Rankin JA Naegel GP Schrader CE Matthay RA Reynolds HY Air-space immunoglobulin production and levels in bronchoalveolar lavage fluid of normal subjects and patients with sarcoidosis.Am Rev Respir Dis. 1983; 127: 442-448Crossref PubMed Scopus (42) Google Scholar A comparable number could be identified among peripheral blood lymphocytes. The actual volume of this immunoglobulin production and its contribution to the level in the BAL fluid are unknown. In certain interstitial lung diseases that are associated with elevated levels of IgG in BAL fluid and serum, however, the number of IgG-secreting lymphocytes is correspondingly increased. Little is known about the half-life and metabolic turnover of IgG in the airway, as have been calculated for IgG and its subclasses in serum.5Morell A Terry WD Waldmann TA Metabolic properties of IgG subclasses in man.J Clin Invest. 1970; 49: 673-680Crossref PubMed Scopus (382) Google Scholar In dogs, the mean half-life of IgG in BAL fluid is 7 ½ days in contrast with a serum half-life of 9 days.31Kazmierowski JA Durbin WA Reynolds HY Kinetics of immunoglobulin transport into canine bronchial secretions.Proc Soc Exp Biol Med. 1976; 152: 493-498Crossref PubMed Scopus (9) Google Scholar In humans, the half-life is about 21 days, although IgG3 is catabolized more quickly (Table 1). Because BAL values of IgG3 and IgG4 in humans are higher than anticipated from respective serum values, these subclasses may be produced in greater amounts locally or their clearance may be prolonged. Undoubtedly, an important function of IgG in the lower respiratory tract is its specific antibody activity against microbial agents or antigens. Although all antibodies belong to one of the classes of immunoglobulins, only a small fraction of antibody actually constitutes the total Ig value; therefore, an appreciable amount of immunoglobulin still has no known specific function. Specific antibodies formed against certain microbial antigens or host proteins (autoantibodies) can be detected in various IgG subclasses (Table 1). IgG2 is especially important as a repository for antibodies against antigens from many common bacteria that cause pneumonia. IgG antibody in the alveolar spaces with specific activity or affinity for a microbe could coat (opsonize) it and then interact with the complement cascade to create a lytic situation that might kill the microbe directly, or the opsonized microbe could be phagocytized by an alveolar macrophage and killed or contained within this cell. A component of complement (C3b) might be used as an additional opsonin to augment membrane receptor attachment to a phagocyte and enhance uptake. Finally, the opsonic antibody, once inside the phagolysosome of a cell, might further aid intracellular killing of the microbe. The relative importance of these options is uncertain; during the inflammatory reaction that occurs with pneumonitis, all are perhaps operative. In the normal lung, the importance of complement activation on the epithelial cell lining surface is questionable. Studies of these interactions might yield some perspective of their perceived importance. Opsonization of microbes or particles is complex, as a mixture of immune opsonins (antibodies) and nonimmune opsonins (surfactant and fibronectin fragments) can be involved.34Czop JK McGowan SE Center DM Opsonin-independent phagocytosis by human alveolar macrophages: augmentation by human plasma fibronectin.Am Rev Respir Dis. 1982; 125: 607-609Crossref PubMed Scopus (20) Google Scholar, 35Jonsson S Musher DM Goree A Lawrence EC Human alveolar lining material and antibacterial defenses.Am Rev Respir Dis. 1986; 133: 136-140Crossref PubMed Scopus (37) Google Scholar, 36Coonrod JD The role of extracellular bactericidal factors in pulmonary host defense.Semin Respir Infect. 1986; 1: 118-129PubMed Google Scholar For example, after a bacterium has entered the alveolar space, it might bump and tumble along the alveolar wall and become enmeshed in the mixture of surfactant and proteinaceous lining fluid and could pick up any or all of these soluble opsonins. Among the immunoglobulins in the alveolar lining fluid, IgM is present in minute amounts,19Waldman RH Jurgensen PF Olsen GN Ganguly R Johnson III, JE Immune response of the human respiratory tract. I. Immunoglobulin levels and influenza virus vaccine antibody response.J Immunol. 1973; 111: 38-41PubMed Google Scholar, 22Reynolds HY Newball HH Analysis of proteins and respiratory cells obtained from human lungs by bronchial lavage.J Lab Clin Med. 1974; 84: 559-573PubMed Google Scholar and IgA is not considered a potent opsonin; thus, IgG is the best candidate. IgM can be an opsonin and fixes complement well, but it has few, if any, membrane receptors for macrophages. In contrast, IgG has specific Fc γ receptors on alveolar macrophages and other phagocytes.37Reynolds HY Atkinson JP Newball HH Frank MM Receptors for immunoglobulin and complement on human alveolar macrophages.J Immunol. 1975; 114: 1813-1819PubMed Google Scholar Phagocytic uptake of viable bacteria, coated with IgG antibody, by alveolar macrophages is increased appreciably in comparison with uptake without an opsonin.38Reynolds HY Kazmierowski JA Newball HH Specificity of opsonic antibodies to enhance phagocytosis of Pseudomonas aeruginosa by human alveolar macrophages.J Clin Invest. 1975; 56: 376-385Crossref PubMed Scopus (53) Google Scholar The opsonizing potential of the individual subclasses, however, could vary substantially on the basis of the relative availability of receptor binding sites on macrophages. Binding with IgG3 (to 30% of cells) and with IgG1 (to about 10% of cells) opsonins occurred on in vitro cultured alveolar macrophages, wh" @default.
• W2084343696 created "2016-06-24" @default.
• W2084343696 creator A5071829879 @default.
• W2084343696 date "1988-02-01" @default.
• W2084343696 modified "2023-09-29" @default.
• W2084343696 title "Immunoglobulin G and Its Function in the Human Respiratory Tract" @default.
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