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In this information, the differences between various kinds of antibodies are explained in detail.

 The specific defence mechanism (immune system) consists of several classes of antibodies, proteins which circulate in the blood. Another name for these antibodies is immunoglobulins (Ig). Examples are IgA, IgE, IgG and IgM. The most important characteristics of a few of these antibodies will be discussed hereafter.

Immunoglobulin A: Another name for IgA is secretory antibody as it is secreted by glandlike cells such as can be found in the bronchi, gastrointestinal tract and vagina. Through the process of binding, it prevents allergens, bacteria and viruses from adhering to the mucous membrane, which could otherwise cause infection or inflammation. Unless there is an IgA shortage, secretory IgA prevents, for example, allergens from being passed to the baby through breast milk.

Immunoglobulin E: IgE is the antibody involved in certain (type I) allergic reactions. Almost all IgE is bound to cells playing a role in allergic reactions, i.e. mast cells and basophil leucocytes. Only a very small amount of IgE circulates in the blood. Whenever an allergen (for example a cow's milk protein molecule) sticks on to the mast cell bound IgE, this results in the secretion of histamine and other mediators, causing an allergic reaction. Histamine circulates in the bloodstream and binds to receptors anywhere in the body. Depending on where binding actually occurs, it leads to allergic asthma, pyrosis or other complaints.

Furthermore, IgE plays a role in the defence against intestinal parasites such as seat worms.

Immunoglobulin G: IgG constitutes the largest class of immunoglobulins: eighty percent of the body's antibody content consists of IgG. In the case of secondary immune reactions (repeated contact) the body switches to the production of IgG. It is important for the protection of the foetus as it can pass through the placenta. IgG is an important antibody in the defence against bacteria and viruses (Roitt, 1988). IgG is divided into four subclasses: IgG1, IgG2, IgG3, and IgG4, each having a different function.

Immunoglobulin M: IgM comprises five monomers and has ten binding sites. It is the first antibody formed in an immune reaction and is mainly involved in reactions against bacteria and viruses. A few days later, IgG will be produced instead of IgM.

Type I allergies include anaphylactic and atopic reactions. Both of these are the result of contact between mast cell bound specific IgE and the corresponding allergen. This leads to mast cell degranulation which triggers the release of numerous vasoactive and chemotactic substances: histamine, heparin, leucothrombin, prostaglandins and thrombosins.

An atopic reaction occurs there where the antigen comes in contact with the mucous membranes, leading only to localized sumptoms such as a running nose in the case of hayfever.

When blood serum is used in allergy testing, it is important to select the class(es) of antibodies to test for. The aim of the test is important in this respect. Naturally, when reactions against allergens are tested for, specific IgE antibodies will be looked for. However, not only IgE antibodies can trigger the release of histamine from mast cells, but also so called "short-term sensitizing" IgG antibodies. These "short-term sensitizing" antibodies are of the G4 class (IgG4) (Turner-Warwick, 1989; Van der Zee et al., 1987).

 In an increasing number of researches a connection is found between IgG4 antibodies and sensitivity reactions. Serious allergic diseases such as atopic eczema, bronchitic asthma and even anaphylactic shock have been found in patients with high IgG4 titres, while no IgE antibodies were found (Halpern, 1987).

Preary (1988) did find IgG4 antibodies in 18 out of 20 patients (90%) who had tested positive for milk in intradermal skintests and elimination and provocation tests, but in whom no IgE specific antibodies against milk had been found.

From a group of 68 atopic children, Nakagawa (1988) selected 18 who suffered from a chicken's egg allergy. These children tested positive in a skin test and in elimination and provocation tests for egg. In only 13 out of 18 children could sufficient IgE specific antibodies against egg be found as explanation for the positive response. In two children, however, very low IgE titres were found; three children lacked these antibodies altogether.

Gwynn et al. (1982), Topping et al. (1989) and Nielsen et al. (1988) proved that, in addition to IgE antibodies, IgG4 antibodies are involved in allergic lung diseases. They demonstrated a connection between the IgG4 antibodies and the patient's exposure to the allergen/antigen. Katila et al. (1986) also found that the concentration of IgG4 antibodies is an indication for exposure to inhaled allergens.

For fungal allergens as well, a connection was discovered between the grade of exposure (consumption) and the IgG4 titre, provided that there is a certain regular and prolonged contact and a certain hereditary disposition.

Increased IgG4 titres are found in IgA deficiency, chronic bowel diseases, malnutrition, food allergy, inhaled allergy (pollenosis), asthma, fungal allergy, atopic eczema, erythema, urticaria, anaphylaxis and cystic fibrosis (Halpern et al., 1987).

The theory is that a patient with allergic symptoms (complaints) will have either high IgE or IgG4 titres against the allergen/antigen. If both concentrations are high, the patient will hardly have any symptoms or none at all, due to the inhibiting effect of IgG4 on IgE (Nakagawa, 1988). In that case a skin test could be negative. This theory is also called the De Weck's theory or hypothesis (De Weck, 1981).

This would explain why patients with allergic complaints are mainly women. Women have significantly lower IgG4 titres than men (Merrett et al., 1983). However, research carried out by Gamboa et al. (1986) does not support the De Weck's hypothesis (see page 6).

Immunoglobulin G comprises four subclasses: IgG1, IgG2, IgG3, and IgG4. These subclasses differ in chemical and biological properties. There are, however, many similarities as well.

Normally, type G4 antibodies occur only in quite small amounts. Only four percent of the IgG in the serum of healthy persons consists of antibodies of this subclass. The concentration (titre) may increase significantly as a result of chronic contact with antigens and then rise to more than 95 percent of the total IgG amount.

Significant IgG4 titres are found especially after prolonged exposure to allergens. Beekeepers who are new to the profession react in the beginning to certain bee venom components by making IgG1 antibodies. After some time, however, a shift occurs towards IgG4 antibodies. Patients with chronic parasitic and fungal infections may also have high IgG4 titres.

IgG4 antibodies have several important properties, which are lacking in other IgG subclasses:

• IgG4 antibodies cannot activate the complement system
• IgG4 cannot bind or activate the first component of the complement system (Clq) (Ishizaka et al., 1967). IgG4 antibodies may play a role, however, in complement activation by other IgG subclasses. In the phospholipid A-antigen system, for example, IgG4 antibodies proved to curb Clq binding and therefore complement activation by IgG1. This probably is a result of competition between IgG1 and IgG4 antibodies for binding sites (determinants) on the antigen (Nakagawa, 1988).
• IgG4 antibodies are functionally monovalent
• IgG4 antibodies can only use one of the two available binding sites. IgG antibodies have two binding sites for antigens. If an antigen is present in an optimum concentration, one antibody can combine with two antigens. In this way a three dimensional network occurs if the antigen has more than two determinants for antigens. The IgG4 antibody also has two binding sites for antigens but is only capable of using one binding site due to three dimensional blocking. IgG4 can therefore not form three dimensional networks with the antigen; immune complexes comprising IgG4 are therefore always quite small and will get trapped in the smallest capillaries.
• IgG4 antibodies can bind to mast cells as well as basophil leucocytes
• When anti IgG4 antibodies (for example from rabbits) are brought into the human bloodstream, histamine is released from basophil leucocytes (Fagan et al., 1982; Van Toorenenbergen et al., 1981). This does not occur in the case of antibodies directed against other IgG antibodies. The release of histamine from basophil leucocytes and mast cells means that IgG4, just as IgE, can bind to these cells. The combination affinity between IgG4 and mast cells is, however, significantly lower than the affinity with IgE.

The precise role of mast cell bound IgG4 is not known yet. There are two hypotheses, however, which explain the working of cell bound IgG4.

IgG4 may, just as IgE, have an anaphylactic effect. In that case, binding of an antigen to mast cell bound IgG4 would have to result in degranulation of the mast cells. This is hard to prove. Another possibility is that cell bound IgG4 modifies or blocks IgE mediated reactions (Nakagawa, 1988). Both hypotheses are discussed in detail hereafter.

Experiments have shown that IgG4 antibodies are bound to mast cells of the skin. If antigen is added, this sticks on to the IgG4 bound to the mast cell, resulting in the release of histamine and sensitization of the skin. This is an anaphylactic reaction.

Anaphylactic IgG4 does, however, have entirely different properties than IgE antibodies:

- it sensitizes the skin faster and for a shorter period than IgE;

- it binds very poorly to cells and tissues;

- it can withstand heat and chemical reactions very well.

 Anaphylactic IgG antibodies are also called "short term sensitizing" IgG, or IgG (S-TS) to make them stand out from IgE antibodies. The incident of these IgG (S-TS) antibodies is usually very low. In a group of 250 asthma patients, for example, only two percent had IgG (S-TS) antibodies in their serum, while 56 percent had a significant IgE titre. In people allergic to milk or grass pollen, the IgG (S-TS) incident is substantially higher.

The hypothesis that IgG4 may act as anaphylactic antibody originates in the discovery that mast cells have IgG4 receptors. The correctness of this hypothesis was supported by the discovery of anaphylactic acting IgG (S-TS) antibody and indications that this antibody belongs to the IgG4 subclass.

These indications are:

• Mast cells of certain individuals have IgG4 receptors;
• From an electrophoretic point of view, IgG4 antibodies are the fastest of all IgG antibodies and cannot activate the complement system. IgE also has these properties;
• In atopic patients a high incident of IgG4 antibodies occurs.
• The assumption that the IgG (S-TS) antibodies belong to the IgG4 sub class is based on the similarities between IgG4 and anaphylactic IgE. However, there are also research results which contradict the assumption that IgG4 may have anaphylactic properties:
• A serological research in people with IgG (S-TS) activity showed that significant IgG4 titres were not always demonstrable;
• It has never been univocally demonstrated that antigenic stimulation of mast cell bound IgG4 results in mast cell degranulation. The IgG (S-TS) antibody does show this degranulation after antigenic stimulation;
• If IgG4 were to show anaphylactic activity this can only be attributed to a small sub-group of the IgG4 class. There are, after all, many examples of persons having high IgG4 titres but no anaphylactic symptoms (Van der Zee et al., 1987).

These counter-arguments do not take into account that a high IgE titre counteracts IgG4 activity. The opposite is also true: a high IgG4 titre can block the IgE effect. The conclusion can therefore only be correct if the research has been carried out in patients having a very low or no IgE response (see hereafter).

IgG4 antibodies can modify IgE mediated reactions by blocking the binding of an allergen to mast cell bound IgE. This was concluded from serological research in patients who had taken immunotherapy (desensitization). The principle of immunotherapy is that allergic individuals are injected with allergens. The result is hypo or desensitization; complete desensitization is hardly ever achieved.

The most important immunological result of hyposensitization is the production of heat stabile antibodies. These antibodies proved to be able to curb IgE mediated reactions. It is likely that this occurs because these antibodies bind to the antigenic determinants on the allergen, as a result of which binding of that allergen to cell bound IgE and consequent degranulation is prevented. These antibodies are called "blocking antibodies".

The majority of blocking antibodies belong to the IgG class. At the beginning of immunotherapy, IgG1 antibodies are produced, but in the course of treatment a shift towards an IgG4 dominated response occurs. Hyposensitization involves, in addition to the production of blocking antibodies, clinical progress of the patient. It seems logical to assume that there is a proportional connection between the concentration of blocking antibodies and the degree of clinical progress. This connection proves to exist exclusively in cases where allergens are involved which enter the bloodstream directly and where the allergic reaction also takes place in the bloodstream.

In practice this means that only in the case of an insect sting allergy the concentration of blocking IgG in the serum is an indication for the degree of clinical progress.

The De Weck's hypothesis (1981) consists of the theory that IgG4 which in low concentrations is mainly bound to mast cells and basophil leucocytes, can cause an allergic reaction, but that in higher concentrations it is mainly not-bound and therefore shows blocking properties.

This latter assumption is in contradiction with research carried out by Gamboa et al. (1986). They found a difference in reaction after bronchial provocation in people with and without specific IgG4. When only IgG4 specific antibodies were present, the patient showed a delayed response, two to four hours after provocation, lasting six to ten hours, wherein all lungfunction parameters decreased. When only IgE specific antibodies were present, the reaction occurred almost immediately after provocation. In the presence of both antibodies both reactions were found, the immediate as well as the delayed reaction. More than half (54%) of all atopic children with a house dust mite allergy have antibodies of the E and G4 class against house dust mite in the blood. Of these atopic children 47 percent shows an immediate as well as a delayed response after broncial provocation. In children with allergic intestinal complaints, this dual response was also found when both antibodies were present. In children with IgE antibodies only, the reaction occurred within twenty minutes to an hour, whereas in children with IgG4 antibodies only, this reaction occurred only four to six hours after the meal.

Research by El Rafei (1989) shows that there is a stronger connection between positive elimination and provocation tests and the occurrence of specific IgE and/or IgG4 antibodies than between these tests and a positive skin test. This connection is even more significant between the occurrence of specific IgE and/or IgG4 antibodies and the anamnesis. El Rafei concludes from this that the patient is very well capable to point out the suspected food and that provocation by only eight gram of food in a capsule in some cases is too little to obtain a positive result, especially in the case of IgG4 antibodies.

In 1987 a congress was organized on the function of IgG4 in allergic reactions. Halpern (1988) summarized the function as follows: "Some people synthesize rather high IgG4 titres against milk and/or egg protein. However, this stimulation varies from individual to individual and has a hereditary basis, much the same as the reaction to injected or inhaled allergens. There appears to be a certain group of people, in whom clinical symptoms coincide with the presence of these IgG4 antibodies. Some children react to milk with an anaphylactic reaction without specific IgE antibodies being demonstrably present." A connection between IgG4 antibodies and symptoms coincides with a decrease in IgG4 antibodies against food allergens after six to twelve weeks of elimination.

In mother's milk, normally fifteen percent of the total amount of IgG is of the G4 type, while this percentage amounts to four, approximately, in the blood serum. These antibodies are directed against a number of food products and inhaled allergens. There is a strong surmise that some patients secrete large amounts of IgG4 specific antibodies in the mammary gland (Halpern, 1987). This occurs especially when there are low concentrations of IgA antibodies, as in the case of atopic women. These high concentrations of antibodies may lead to allergic reactions in breastfed babies.

In a patient with asthmatic complaints, brought about by the consumption of a chicken's egg, Nakagawa (1988) proved that a type III allergic reaction occurred, involving a strong increase in the blood of IgG1 specific antibodies against egg within fifteen minutes. The lungfunction (FEV1) of the patient showed a strong decrease at the same time. After six hours, the patient suffered from shortness of breath, wheezing and tightness of the chest (dyspnoea).

In patients with inflammatory bowel diseases such as ulcerative colitis, only highly increased IgG1 titres were found (Scott et al., 1986). A connection was found between complaints and specific IgG1 antibodies. With these syndromes it is of more use to determine the specific IgG or IgG1 antibodies than IgG4.

Increased IgG1 titres are furthermore found in rheumatoid arthritis, lupus erythematosus, glomerulonephritis and certain bowel diseases (Shakib et al., 1980; Oxelius, 1984; Heiner, 1984). In the case of Crohn's disease, all IgG subclasses show a slight increase.

More than 65 percent of the IgG in the serum of healthy people consists of IgG1. Through exposure to specific antigens this may increase to 98 percent. The concentration in the serum normally amounts to approximately 8 mg/ml. IgG1 has other properties than IgG4:

• It is suspected that IgG1 antibodies cannot bind to mast cells
• Binding of IgG antibodies to mast cells is a sole property of IgG4. IgG1 antibodies cannot bind to mast cells and can therefore not cause a type I allergic reaction.
• IgG1 antibodies are functionally bivalent
• IgG1 antibodies can, contrary to IgG4 antibodies, use both binding sites. IgG1 antibody-antigen complexes are therefore quite large in size.
• IgG1 antibodies can activate the complement system

IgG1 antibody activity often works mainly through type III allergic reactions and often results in inflammatory responses in organs. The most common symptoms are nephritis, rheumatoid arthritis, alveolitis and vasculitis.

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