出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2016/08/23 22:07:24」(JST)
免疫グロブリンG(めんえきグロブリンG、Immunoglobulin G、IgG)は単量体型の免疫グロブリンで、2つの重鎖γと2つの軽鎖からなっている。それぞれの複合体は2つずつの抗原結合部位を持っている。免疫グロブリンの中では最も数の多いものである。ヒトの血清の免疫グロブリンの75%を占め[1]、体中の血液、組織液に存在する。
鳥類のIgGはしばしばIgYと呼ばれ、血清と卵黄の中に見られる[2]。
IgGはヒトの胎盤を通過できる唯一のアイソタイプであり、自分の免疫系を確立する生後1週間までの間、胎児を守っている。
IgGはウイルス、細菌、真菌など様々な種類の病原体と結合し、補体、オプソニンによる食作用、毒素の中和などによって生体を守っている。
特異的IgGは食物アレルギーでない人にも存在し、食物アレルギーの診断における有用性は確立されていない
ヒトの場合
種類 | 割合 | 胎盤の通過 | 補体の活性化 | 食細胞のFc部位への結合 |
IgG1 | 66% | する | 2番目に高い | 高い |
IgG2 | 23% | しない | 3番目に高い | 極めて低い |
IgG3 | 7% | する | 最も高い | 高い |
IgG4 | 4% | する | しない | 中程度 |
動物種によってサブクラスの数は異なる。
|
This article may be too technical for most readers to understand. Please help improve this article to make it understandable to non-experts, without removing the technical details. The talk page may contain suggestions. (January 2015) (Learn how and when to remove this template message) |
Immunoglobulin G (IgG) is a type of antibody. It is a protein complex composed of four peptide chains—two identical heavy chains and two identical light chains arranged in a Y-shape typical of antibody monomers. Each IgG has two antigen binding sites. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in the circulation.[1] IgG molecules are created and released by plasma B cells.
Antibodies are major components of humoral immunity. IgG is the main type of antibody found in blood and extracellular fluid allowing it to control infection of body tissues. By binding many kinds of pathogens such as viruses, bacteria, and fungi, IgG protects the body from infection.
It does this through several mechanisms:
IgG antibodies are generated following class switching and maturation of the antibody response and thus participate predominantly in the secondary immune response.[3] IgG is secreted as a monomer that is small in size allowing it to easily perfuse tissues. It is the only isotype that has receptors to facilitate passage through the human placenta, thereby providing protection to the fetus in utero. Along with IgA secreted in the breast milk, residual IgG absorbed through the placenta provides the neonate with humoral immunity before its own immune system develops. Colostrum contains a high percentage of IgG, especially bovine colostrum. In individuals with prior immunity to a pathogen, IgG appears about 24–48 hours after antigenic stimulation.
Therefore, in the first six months of life, the fetus has the same antibodies as the mother, till the old antibodies are then degraded and he can defend himself against all the pathogens that the mother encoutered in her life (even if only through vaccination). This repertoire of immunoglobulins is crucial for the newborns who are very sensitive to infections above all for the respiratory and digestive systems.
It is also demonstrated that in adults, the production of IgG is linked to the nutritional prevalence of a specific food or food-group in the personal diet.[4] Therefore, the levels of IgG for food reflect dietary intake or express a possible previous immune contact with food. The total IgG values towards specific foods indicate excessive or repetitive consumption of them.[5][6] IgG are also involved in the regulation of allergic reactions. According to Finkelman, there are two pathways of systemic anaphylaxis:[7][8] antigens can cause systemic anaphylaxis in mice through classic pathway by cross-linking IgE bound the mast cell FcεRI, stimulating histamine and PAF release. In the Alternative pathway antigens form complexes with IgG that cross-link macrophage Fc γ RIII, stimulating only PAF release.
IgG antibodies can prevent IgE mediated anaphylaxis by intercepting a specific antigen before it binds to mast cell –associated IgE. Consequently, IgG antibodies block systemic anaphylaxis induced by small quantities of antigen but can mediate systemic anaphylaxis induced by larger quantities.
IgG antibodies are large molecules of about 150[9][10] kDa made of four peptide chains. It contains two identical class γ heavy chains of about 50 kDa and two identical light chains of about 25 kDa, thus a tetrameric quaternary structure. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves, which together form the Y-like shape. Each end of the fork contains an identical antigen binding site. The various regions and domains of a typical IgG are depicted in the figure to the left. The Fc regions of IgGs bear a highly conserved N-glycosylation site. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6-linked sialic acid residues.[11]
There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).
Name | Percent | Crosses placenta easily | Complement activator | Binds to Fc receptor on phagocytic cells | Half Life[12] |
IgG1 | 66% | yes (1.47)* | second-highest | high affinity | 21 days |
IgG2 | 23% | no (0.8)* | third-highest | extremely low affinity | 21 days |
IgG3 | 7% | yes (1.17)* | highest | high affinity | 7 days |
IgG4 | 4% | yes (1.15)* | no | intermediate affinity | 21 days |
* Quota cord/maternity concentrations blood. Based on data from a Japanese study on 228 mothers.[13] |
Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. The structure of the hinge regions (region 6 in the diagram) contributes to the unique biological properties of each of the four IgG classes. Even though there is about 95% similarity between their Fc regions, the structure of the hinge regions is relatively different.
Given the opposing properties of the IgG subclasses (fixing and failing to fix complement; binding and failing to bind FcR), and the fact that the immune response to most antigens includes a mix of all four subclasses, it has been difficult to understand how IgG subclasses can work together to provide protective immunity. Recently the Temporal Model of human IgE and IgG function was proposed.[14] This model suggests that IgG3 (and IgE) appear early in a response. The IgG3, though of relatively low affinity, allows IgG-mediated defences to join IgM-mediated defences in clearing foreign antigens. Subsequently, higher affinity IgG1 and IgG2 are produced. The relative balance of these subclasses, in any immune complexes that form, helps determine the strength of the inflammatory processes that follow. Finally, if antigen persists, high affinity IgG4 is produced, which dampens down inflammation by helping to curtail FcR-mediated processes.
The relative ability of different IgG subclasses to fix complement may explain why some anti-donor antibody responses do harm a graft after organ transplantation.[15]
In a mouse model of autoantibody mediated anemia using IgG isotype switch variants of an anti erythrocytes autoantibody, it was found that mouse IgG2a was superior to IgG1 in activating complement. Moreover, it was found that the IgG2a isotype was able to interact very efficiently with FcgammaR. As a result, 20 times higher doses of IgG1, in relationship to IgG2a autoantibodies, were required to induce autoantibody mediated pathology.[16] It is important to remember that mouse IgG1 and human IgG1 are not necessarily similar in function, and the inference of human antibody function from mouse studies must be done with great care. Nevertheless, it remains true that both human and mouse antibodies have different abilities to fix complement and to bind to Fc receptors.
The measurement of immunoglobulin G can be a diagnostic tool for certain conditions, such as autoimmune hepatitis, if indicated by certain symptoms.[17] Clinically, measured IgG antibody levels are generally considered to be indicative of an individual's immune status to particular pathogens. A common example of this practice are titers drawn to demonstrate serologic immunity to measles, mumps, and rubella (MMR), hepatitis B virus, and varicella (chickenpox), among others.[18]
Testing of IgG is not indicated for diagnosis of allergy.[19][20]
The levels of IgG towards foods reflect nutritional exposure and indicate excessive or repetitive consumption of them.[5][6] According to a satistical analysis done at Duke University by Soriano et al.[21] measuring IgG levels for food allows clinicians to aggregate their values and to define the nutritional prevalence of the “Great Food Groups” in a personal diet, suggesting a “personal profile” specific for each person.[22] This approach refuses any correlation with allergy and is a complete novelty in comparison to several tests looking for reactivities to single foods.
The immune system recognises foods by IgG not with a strong specificity, as is the case for IgE, but rather by an approach of similarity. This, on one hand, determines a greater capacity to recognise foods and maintain tolerance and, on the other hand, determines the possibility of inducing inflammation not only with foods that are eaten more frequently but also with foods that the immune system recognises as similar to those.[23]
IgG antibodies are extracted from donated blood plasma and used as a therapeutic known as intravenous immunoglobulin (IVIG). This is used to treat immune deficiencies, autoimmune disorders, and infections.
Antibodies of the IgG class are predominantly active during a secondary antibody response. Thus, the appearance of specific IgG antibodies generally corresponds with the 'maturation' of the antibody response, which is switched on upon repeated contact with an antigen.
Immunology: lymphocytic adaptive immune system and complement
|
|||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Lymphoid |
|
||||||||||||||
Lymphocytes |
|
||||||||||||||
Substances |
|
Antibodies
|
|
---|---|
Antibodies |
|
see also disorders of globin and globulin proteins
|
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
国試過去問 | 「114D021」「109I057」「109A039」「113A057」「109D048」「104E053」「109A036」「112A060」「106D051」「105I055」「103E045」「110E037」「098G044」「100G051」「109D004」「107D010」「102I006」「108E032」「109B022」「106B030」 |
リンク元 | 「神経性食思不振症」「自己免疫性肝炎」「免疫グロブリン」「アレルギー」「膜性増殖性糸球体腎炎」 |
C
※国試ナビ4※ [114D020]←[国試_114]→[114D022]
B
※国試ナビ4※ [109I056]←[国試_109]→[109I058]
B
※国試ナビ4※ [109A038]←[国試_109]→[109A040]
A
※国試ナビ4※ [113A056]←[国試_113]→[113A058]
B
※国試ナビ4※ [109D047]←[国試_109]→[109D049]
※国試ナビ4※ [104E052]←[国試_104]→[104E054]
B
※国試ナビ4※ [109A035]←[国試_109]→[109A037]
B
※国試ナビ4※ [112A059]←[国試_112]→[112A061]
C
※国試ナビ4※ [106D050]←[国試_106]→[106D052]
※国試ナビ4※ [105I054]←[国試_105]→[105I056]
CD
※国試ナビ4※ [103E044]←[国試_103]→[103E046]
DE
※国試ナビ4※ [110E036]←[国試_110]→[110E038]
E
※国試ナビ4※ [098G043]←[国試_098]→[098G045]
B
※国試ナビ4※ [100G050]←[国試_100]→[100G052]
D
※国試ナビ4※ [109D003]←[国試_109]→[109D005]
A
※国試ナビ4※ [107D009]←[国試_107]→[107D011]
A
※国試ナビ4※ [102I005]←[国試_102]→[102I007]
AD その他の正答:DE、AE、
※国試ナビ4※ [108E031]←[国試_108]→[108E033]
D
※国試ナビ4※ [109B021]←[国試_109]→[109B023]
E
※国試ナビ4※ [106B029]←[国試_106]→[106B031]
血算 | ヘモグロビン | 減少(貧血) |
白血球 | 減少 | |
リンパ球 | 比較的増加 | |
生化学 | Na | 低Na血症(自己嘔吐・下剤使用例) |
K | 低Ka血症(自己嘔吐・下剤使用例) | |
AST | 上昇 | |
ALT | 上昇 | |
LDH | 上昇 | |
T-Cho | 上昇 | |
血糖 | 低下 | |
血清学 | IgG | 低下(易感染性はない) |
内分泌 | T3 | 低下 |
reverse T3 | 上昇 | |
GH | ↑ | |
LH | ↓ | |
FSH | → or ↑ | |
コルチゾール | → or ↑ |
神経性食思不振症 | 下垂体機能低下症 | ANについて | |||
好発年齢 | 思春期 | 全年齢 | |||
性差 | 女>>男 | なし | |||
体重 | 著明に減少 | 不定 | |||
食欲不振 | 高度 | 希 | |||
食行動の異常 | 高頻度 | なし | |||
精神運動異常 | 活発 | 無欲状 | |||
月経異常 | あり(体重減少に先行) | あり | |||
産毛の増加 | あり | なし? | |||
恥毛・腋毛脱落 | なし | あり | LH, FSHが少しは存在するため。 | ||
乳腺萎縮 | 軽度 やせに比して乳房は保たれる |
著明 | |||
下垂体機能 | GH | 正常~高値 | 低値 | IGF-I低値 | |
ACTH | 正常~高値 | 低値 | |||
LH, FSH | 低値 | 低値 | |||
TSH | 正常 | 低値 | |||
甲状腺機能 | 低T3症候群 | 機能低下 | T4正常、reverse T3上昇 | ||
副腎皮質機能 | 正常 (コルチゾール高値例あり) |
機能低下 (コルチゾール低値) |
|||
頭部CT・MRI | 異常なし | 異常例有り (下垂体腫瘍, empty sellaなど) |
|||
症状 | 背部のうぶ毛の増加、便秘、 低血圧、徐脈、下腿浮腫、 循環障害による皮膚色の変化や 凍瘡、末梢神経麻痺、 カロチン症など |
AIH | 自己抗体 | HCV感染 | |||
抗核抗体 | 抗平滑筋抗体 | 抗LKM-1抗体 | 抗SLA抗体 | ||
ANA | ASMA | ||||
I型 | + | + | - | - | - |
IIa型 | - | - | + | - | - |
IIb型 | - | - | + | - | + |
III型 | - | - | - | + | - |
IV型 | - | + | - | - | - |
1. 血中自己抗体(特に抗核抗体、抗平滑筋抗体など)が陽性。 |
2. 血清γグロブリン値またはIgGの上昇 (2g/dl以上)。 |
3. 持続性または反復性の血清トランスアミナーゼ値の異常。 |
4. 肝炎ウィルスマーカーは原則として陰性。 |
5. 組織学的には肝細胞壊死所見およびpiecemeal necrosisに伴う慢性肝炎あるいは肝硬変であり、しばしば著明な形質細胞浸潤を認める。時に急性肝炎像を呈する。 |
註 * 本邦ではHLA-DR4陽性症例が多い ** 本邦ではC型肝炎ウィルス血症を伴う自己免疫性肝炎がある。 *** C型肝炎ウィルス感染が明らかな症例では、インターフェロン治療が奏功する例もある。 |
抗体 | IgG1 | IgG2 | IgG3 | IgG4 | IgM | IgA1 | IgA2 | IgD | IgE |
重鎖 | γ1 | γ2 | γ3 | γ4 | μ | α1 | α2 | δ | ε |
分子量 | 146 | 146 | 165 | 146 | 970 | 160 | 160 | 184 | 188 |
補体活性化(古典的経路) | ++ | + | +++ | - | ++++ | - | - | - | - |
補体活性化(代替経路) | - | - | - | - | - | + | - | - | - |
胎盤通過 | +++ | + | ++ | ± | - | - | - | - | - |
食細胞FcRへの結合 | + | - | + | ± | - | + | + | - | + |
肥満細胞・好塩基球への結合 | - | - | - | - | - | - | - | - | +++ |
staphylococcal Protein Aとの反応性 | + | + | ± | + | - | - | - | - | - |
IgG | IgA | IgM | ||||
♂ | ♀ | ♂ | ♀ | ♂ | ♀ | |
1ヶ月 | 400 ~ 1030 | ー ~ 24 | 21 ~ 96 | |||
6ヶ月 | 290 ~ 950 | 8 ~ 50 | 46 ~ 176 | |||
1歳 | 460 ~ 1220 | 470 ~ 1210 | 16 ~ 128 | 14 ~ 98 | 57 ~ 260 | 81 ~ 314 |
3歳 | 530 ~ 1340 | 540 ~ 1340 | 25 ~ 174 | 22 ~ 150 | 63 ~ 279 | 86 ~ 332 |
6歳 | 630 ~ 1490 | 650 ~ 1530 | 45 ~ 258 | 38 ~ 238 | 72 ~ 305 | 92 ~ 353 |
12歳 | 750 ~ 1660 | 790 ~ 1740 | 71 ~ 352 | 63 ~ 373 | 72 ~ 306 | 100 ~ 380 |
成人 | 680 ~ 1620 | 84 ~ 438 | 380 ~ 1620 |
I型アレルギー | II型アレルギー | III型アレルギー | IV型アレルギー | V型アレルギー | |||
免疫反応の主体 | IgE | IgM, IgG | 免疫複合体 | Th1細胞 | Th2細胞 | Tc細胞 | IgG |
抗原 | 水溶性抗原 | 細胞や マトリックスに 結合している抗原 |
水溶性抗原 | 水溶性抗原 | 水溶性抗原 | 細胞関連の抗原 | 細胞表面レセプター |
エフェクター機構 | 肥満細胞の活性化 | 補体 (CDC)|NK細胞、好中球 (ADCC) | 補体、好中球 | マクロファージの活性化 | IgE産生、好酸球と肥満細胞活性化 | 細胞障害 | 抗体の結合 |
アレルギー疾患の例 | 気管支喘息 アレルギー性鼻炎 花粉症 喘息 蕁麻疹 アトピー性皮膚炎 ラテックスアレルギー |
ABO不適合輸血 新生児溶血性貧血 グッドパスチャー症候群 自己免疫性溶血貧血 AIHA 特発性血小板減少性紫斑病 ITP 橋本病 超急性移植片拒絶反応 |
アルツス反応 アニサキス症 トリ飼い病 農夫肺 血清病 急性糸球体腎炎 ループス腎炎(SLE) 関節リウマチ 過敏性肺臓炎 アレルギー性気管支肺アスペルギルス症 |
接触性皮膚炎 ツベルクリン反応 慢性肉芽腫症 |
慢性喘息 慢性アレルギー鼻炎 |
移植片拒絶 | 慢性蕁麻疹 バセドー病 重症筋無力症 |
検査方法 | RAST法 RIST法 皮内反応 プリックテスト スクラッチテスト 負荷試験 |
免疫組織染色 凝集反応(完全抗体) 凝集反応(不完全抗体, クームステスト) |
皮内反応 |
.