Histamine-releasing Autoantibodies Research Articles

Histamine-release test in angioedema patients without urticaria - a retrospective cohort study of 404 patients.

A subset of patients with angioedema (AE) and urticaria has histamine releasing autoantibodies. The histamine release test (HR-test) has been used as a tool in chronic urticaria to define the autoimmune subgroup and may possibly guide the clinician to a more personalized therapy, like omalizumab and cyclosporine. The prevalence and value of positive histamine releasing autoantibodies in monosymptomatic AE is sparsely described in the literature. The purpose of this study was to report the prevalence of positive histamine releasing autoantibodies in a cohort of patients with recurrent AE and evaluate the usefulness of this test in AE patients. We performed a retrospective cohort study of 612 patients referred due to AE between 1995 and 2013. HR-test results were available in 404 patients. In the sub-group of patients with AE and urticaria, 17.3% had a positive HR-test but only 4.3% of patients with mono-symptomatic AE had a positive HR-test. No statistically significant treatment benefits of antihistamines, corticosteroids or adrenaline were found comparing patients with angioedema +/- urticaria based on the result of the HR-test (negative / positive). Thus, the HR-test result cannot be used as predictor of the efficacy of anti-allergic treatment.

Correlation of serum antithyroid microsomal antibody and autologous serum skin test in patients with chronic idiopathic urticaria.

Background:About 25–45% of patients of chronic urticaria (CU) have been stated to have histamine releasing autoantibodies in their blood. The term autoimmune urticaria is increasingly being accepted for this subgroup of patients. Review of the literature suggests high autologous serum skin test (ASST) positivity and presence of antithyroid microsomal antibodies in patients with autoimmune urticaria.Aims:To study prevalence of ASST positivity and antithyroid microsomal antibodies in chronic “idiopathic” urticaria and to study the correlation between the two parameters.Methods:All patients of chronic idiopathic urticaria satisfying inclusion/exclusion criteria were enrolled in the study after written informed consent. Patients of CU secondary to infections and infestations, physical urticaria including dermatographism, mastocytosis, urticarial vasculitis and those on treatment with immunosuppressive drugs for urticaria were excluded from the study. In all of these patients, complete blood count; ASST, serum T3/T4/thyroid stimulating hormone levels, antithyroid microsomal antibody (AMA) levels were done. Statistical analysis was done by Chi-square test, Fisher exact test and Kappa statistics.Results:Study included 24 males and 26 females with mean age of 39.54 years. Majority of patients belonged to 20–40 years of age. Females showed more ASST positivity. A total of 12 out of 50 (24%) patients showed positive ASST. A total of four out of 12 (33.33%) had positive ASST and raised AMA levels.Conclusion:Only 25% of patients of chronic idiopathic urticaria had positive ASST. ASST and AMA levels were positively correlated in our study. Further studies are required to authenticate this association.

Pathomechanisms of Chronic Spontaneous Urticaria: What Is Known and Up to Date

The pathomechanism of chronic spontaneous urticaria (CSU), defined by the recurrence of short-lived wheals for more than 6 weeks without apparent triggers, remained to be mysterious until 1986. In that year, skin reactivity to the injection of autologous serum was reported in about half of patients with CSU; that observation and the subsequent discovery of histamine-releasing autoantibodies against the high-affinity IgE receptor (FceRI) and against IgE in patients with CSU were major breakthroughs in the pathogenesis of this disorder. A role for complement activation in autoantibody-dependent histamine release was subsequently demonstrated. CSU was hence considered as an autoimmune disorder due to histamine-releasing autoantibodies in a consistent proportion of cases. However, this model does not fit all cases and shows several inconsistencies. Recently, it was shown that sera from most CSU patients are able to degranulate also mast cells lacking FceRI, suggesting the involvement of other as yet unknown mechanisms. New light on CSU pathogenesis was shed by the demonstration of coagulation activation and eosinophil involvement during active phases of the disease, paralleling urticaria severity. Finally, the efficacy of omalizumab (a recombinant humanized monoclonal IgG anti-IgE antibody) in the treatment of refractory CSU opened a new scenario in the pathophysiology of the disease. Indeed, omalizumab has multiple biological effects relevant in CSU pathogenesis, such as binding of free IgE, downregulation of mast cell function including expression of high affinity IgE receptors, and induction of eosinophil apoptosis.

Chronic Spontaneous Urticaria: the Emerging Role of Coagulation

Chronic urticaria (CU) is a cutaneous disease characterized by the recurrent eruption of short-lived wheals for at least six weeks. The wheals, which are surface swelling of the dermis accompanied by itching, may be elicited by different stimuli, or occur spontaneously. The pathogenesis of spontaneous CU has long remained a mystery, with food allergy, intolerance to food additives and chronic infections having been indicated as the main causes of the disease. The demonstration of histamine releasing autoantibodies in the serum of CU patients led to consider it of autoimmune origin in about 45 % of cases, this condition being “idiopathic” in the remaining 55 %. We recently evaluated the possible involvement of blood coagulation in its pathophysiology by measuring in CU patients the plasma levels of prothrombin fragment F1 + 2, a polypeptide released into the circulation during the activation of prothrombin to thrombin, and D-dimer, a marker of fibrin lysis. Interestingly, both F1 + 2 and D-dimer plasma levels were extremely high during the acute phase of disease, but were completely normal after remission, paralleling urticaria severity. It is noteworthy that thrombin causes an increase in vascular permeability, thus amplifying the CU inflammatory network. Moreover, immunohistochemical studies showed expression of tissue factor, the main initiator of coagulation, in CU skin lesions, and co-localization experiments demonstrated that tissue factor is expressed by eosinophils in the inflammatory infiltrate. All these findings support that coagulation activation contributes in the pathophysiology of spontaneous CU, and provide the rationale for using also anticoagulant and antifibrinolytic agents to treat it.

Activation of blood coagulation in chronic urticaria: pathophysiological and clinical implications by Dr. Massimo Cugno et al.

Cugno et al. [1] reviewing the topic of chronic urticaria (CU) highlight the role of blood coagulation in the pathophysiology of this disease. This is an innovative view that represents the major breakthrough of the last few years to unravel the poorly understood mechanisms underlying the pathogenesis of urticaria lesions. Previous important discovers in this field are depicted in a figure of the paper that reports the chronology of the key events of CU pathophysiology investigation in the last two decades. The first important discover dates back to the 1980s, when Grattan et al. described the possible autoimmune origin of the disease in about half of CU patients in whom the intradermal injection of autologous serum caused a wheal and flare reaction. Subsequently, the presence of histamine-releasing autoantibodies directed against IgE or the alpha-subunit of the high-affinity IgE receptor (FceRI) was found in about 30–35% of CU patients. The skin reaction to autologous serum was ascribed to the presence of functionally active autoantibodies to FceRI and to other less-characterized histamine-releasing factors as activation of the complement system by IgG autoantibodies against the alpha-subunit of the FceRI that has been shown to enhance histamine release. The autoimmune background, in a consistent proportion of CU patients, is further supported by the frequent association with autoimmune thyroiditis [2] and by anecdotal reports of the association between CU and other autoimmune disorders, such as Graves disease, type 1 diabetes and celiac disease. Although the presence of histamine-releasing autoantibodies accounts for CU pathomechanism in almost half of the patients, the pathomechanisms involved in the other half of patients remain not determined. The studies on blood coagulation can partially fill this gap. Hence, it is known that activation of the coagulation cascade leads to generation of vasoactive substances such as thrombin which stimulate endothelium to increase vascular permeability [3, 4]. CU patients show an activation of coagulation cascade through the tissue factor expressed by eosinophils infiltrating CU lesional skin. Thus, it is possible to hypothesize that the eosinophil is a leading actor in the CU scenario. The activation of coagulation has been described also in other diseases characterized by inflammation, including other cutaneous disorders, such as bullous pemphigoid and angioedema due to C1 inhibitor deficiency [5–8], and a wide spectrum of systemic inflammatory diseases, such as rheumatoid arthritis [9] inflammatory bowel diseases [10] and sepsis [11]. This should not necessarily lead to consider activation of the coagulation as a nonspecific phenomenon, but rather to think that different diseases share, at different sites, a common intermediate step in their pathophysiology. Thus, in the skin microenvironment of CU and bullous pemphigoid, eosinophils express TF which activate coagulation and generate vascular permeability mediators. In angioedema due to C1-inhibitor deficiency, another skin disease characterized by increase in vascular permeability, there is an activation of blood coagulation [7, 8], but edema is mediated by bradykinin generated upon activation of the contact system. An obvious concern with these views comes from the observation that markers of thrombin generation like F1 ? 2 are increased in several prothrombotic conditions, such as disseminated intravascular coagulation [12], deep venous thrombosis [13] and endotoxinemia [14] that are not characterized by urticaria or edema. Therefore, we can conclude that thrombin M. Cicardi (&) Dipartimento di Scienze Cliniche ‘‘Luigi Sacco’’, Universita di Milano, Ospedale L. Sacco Milano, Milan, Italy e-mail: marco.cicardi@unimi.it

Activation of blood coagulation in chronic urticaria: pathophysiological and clinical implications

Chronic urticaria (CU) is a skin disorder characterized by the recurrent eruption of short-lived wheals accompanied by redness and itching for at least 6 weeks. The wheals can be associated with angioedema. CU is considered an autoimmune disease in about 50% of cases with the presence of circulating histamine releasing autoantibodies mainly directed against the high affinity IgE receptor FcepsilonRI on mast cells and basophils or against IgE. In several CU cases regarded as idiopathic; the actual pathophysiological mechanisms are still unknown. Some patients with CU do not respond to antihistamines and require the use of systemic steroids or cyclosporin, which are, however, not always effective. In CU, several investigators have demonstrated the activation of coagulation that is due to the involvement of eosinophils and a tissue factor pathway with generation of thrombin potentially contributing to an increased vascular permeability. CU patients often present with elevation of coagulation and fibrinolysis markers, such as prothrombin fragment F1+2 and D: -dimer, which correlate with the disease severity. Preliminary data indicate that anticoagulant treatment with heparin and warfarin may be effective in reducing the symptoms of this disorder. Taken together, all these findings provide the rationale for proposing clinical trials on the use of anticoagulant drugs as adjuvant treatment in CU patients.

Plasma levels and skin‐eosinophil‐expression of vascular endothelial growth factor in patients with chronic urticaria

Although chronic urticaria (CU) is often regarded as autoimmune in nature, only less than 50% of sera from CU patients contain histamine-releasing autoantibodies. This suggests that other factors may contribute to its pathogenesis. We evaluated the possible involvement of vascular endothelial growth factor (VEGF), one of the major mediators of vascular permeability, in CU. Eighty consecutive adult patients with CU and 53 healthy subjects were studied. VEGF and prothrombin fragment F(1+2) were measured by enzyme immunoassays. Autologous plasma skin test (APST) was performed in CU patients and, in six of them, skin biopsy specimens were taken from wheals to evaluate the immunohistochemical expression of VEGF and eosinophil cationic protein (ECP). Plasma VEGF concentrations were higher in CU patients (8.00 +/- 0.90 pmol/l) than in controls (0.54 +/- 0.08 pmol/l) (P = 0.0001) and tended to parallel both the severity of CU and to correlate with F(1+2) levels. APST was positive in 85.1% of patients. VEGF concentration was significantly higher in APST-positive than in APST-negative patients (P = 0.0003). Immunohistochemically, all specimens from patients with CU showed a strong expression of VEGF (P = 0.002) that colocalized with ECP, a classic eosinophil marker. VEGF plasma levels are elevated in CU and parallel the disease severity. This supports a possible role of this molecule in CU pathophysiology. Eosinophils are the main cellular source of VEGF in CU lesional skin.

The autologous serum skin test may be used as a marker for histamine releasing autoantibodies in urticaria and is not relevant to other subject groups

The autologous serum skin test may be used as a marker for histamine releasing autoantibodies in urticaria and is not relevant to other subject groups

Expression of Tissue Factor by Eosinophils in Patients with Chronic Urticaria

Background: Although several cases of chronic urticaria (CU) are currently regarded as autoimmune in origin, associated with histamine-releasing autoantibodies, an activation of blood coagulation via tissue factor (TF) and a strong expression of TF in lesional skin have been described. Eosinophils, which are involved in CU skin lesions, have recently been demonstrated as the major source of TF in human blood. We assessed whether eosinophils are the cellular source of TF in CU skin lesions. Methods: Twenty patients with severe CU were studied. Skin biopsy specimens were taken from wheals. The control group consisted of specimens of perilesional normal skin from different types of skin tumours (10) and various skin disorders with non-eosinophilic infiltrates, including leukocytoclastic vasculitis (7), lichen planus (8) and mastocytosis (3). TF expression was evaluated by immunohistochemical methods using an anti-TF monoclonal antibody. Co-localization of TF and eosinophil cationic protein, a classic cell marker of eosinophils, was investigated by double-staining studies using 2 specific monoclonal antibodies in the 4 specimens showing the highest TF reactivity scores. Results: All specimens from patients with CU clearly showed TF expression that was absent in all normal control specimens (p = 0.0001) and in the skin disorders with non-eosinophilic infiltrates (p = 0.001–0.0001). The double-staining experiments for TF and eosinophil cationic protein clearly showed that the TF-positive cells were eosinophils. Conclusions: Eosinophils are the main source of TF in CU lesional skin. This finding highlights the role of these cells in the pathophysiology of CU and might pave the way for new therapeutic strategies.

Chronic idiopathic urticaria with functional autoantibodies: 12 years on

It is now recognized that approximately one-third of patients with chronic idiopathic urticaria (CIU) have histamine-releasing autoantibodies directed against either the high-affinity IgE receptor or, less frequently, against IgE. However, there are several unsolved problems relating to the role of such autoantibodies in the disease. Additionally, it is not clear whether CIU with autoantibodies can be classified as an autoimmune disease. The detection of patients with autoantibodies also poses challenges. Firstly, the only in vivo method, the autologous serum skin test, is at best 80% sensitive and specific using in vitro basophil histamine release assays as the verum. Secondly, in vitro tests are only done in a small number of research laboratories, and are not widely commercially available, and thirdly, there is some divergence between results obtained by different methods (functional and immunoassays) used to detect patients with autoantibodies. The presence of autoantibodies may be important clinically in a small group of severely affected, treatment-resistant patients, where immunomodulatory treatments may be helpful.

Autologous serum skin test reactivity in patients with non‐allergic asthma

Inflammatory alterations of respiratory airways have been found in patients with non-allergic asthma, but the triggering event has not been defined. An autoimmune activation of inflammatory cells has been hypothesized. To evaluate whether histamine-releasing factors are present in sera from non-allergic asthmatics. Twenty-four patients with non-allergic asthma underwent in vivo autologous serum skin test (ASST) and in vitro basophil histamine release assay using autologous basophils as well as basophils from normal donors. Twenty-seven subjects with respiratory allergy and three normal subjects were chosen as control. ASST was positive in 14/24 non-allergic asthmatics (58%) whereas it was negative in all 30 control subjects (P<0.001). The serum of only one ASST-positive patient out of 12 (8.4%) induced in vitro histamine release from autologous basophils. The serum from another ASST-positive patient induced histamine release from membrane IgE-stripped autologous basophils. Sera from either non-allergic asthmatics or from control subjects did not provoke significant histamine release from basophils from three normal donors. Skin reactivity to autologous serum is common among non-allergic asthmatics, indicating the presence of circulating histamine-releasing factors. However, only in a minority of patients in vitro functional evidence of histamine-releasing autoantibodies (anti-FcepsilonRI or anti-IgE) was obtained. The presence of circulating histamine-releasing factors might contribute to initiation/maintenance of inflammation in respiratory airways of non-allergic asthmatics.

Chronic urticaria: Expanding the autoimmune kaleidoscope

Chronic urticaria: Expanding the autoimmune kaleidoscope

Blood basophil numbers in chronic ordinary urticaria and healthy controls: diurnal variation, influence of loratadine and prednisolone and relationship to disease activity.

The basopenia of chronic urticaria relates to histamine releasing autoantibodies in the serum of patients with autoimmune urticaria. This reduction in circulating basophils may be due to active recruitment into weals. If so, it might be expected that numbers in blood would be reduced when urticaria is active and increased after treatment. The primary aim of this study was to look at diurnal variation of basophil numbers in patients with chronic ordinary urticaria (not physical or vasculitic) in relation to disease activity and the effect of treatment with antihistamines and corticosteroids, and to compare the results with healthy controls. A secondary aim was to compare a standard manual counting method with automated basophil counts and to look at numbers of other circulating leucocytes that might be relevant to urticaria pathogenesis. Manual basophil counts using a toluidine blue stain and automated 5-part differentials (Coulter Gen. S) were performed at 4-hourly intervals from 08.00 to 20.00 in 10 healthy controls (six women, age 24 to 63 years) and seven chronic urticaria patients (five women, 24 to 50 years). All chronic urticaria patients had severe daily or almost daily urticaria. Only one of six chronic urticaria sera showed in vitro basophil histamine releasing activity. Counts were performed without treatment, after a week of taking loratadine 10 mg daily and after 3 days of adding prednisolone at 0.6 mg/kg/day (maximum 40 mg). Daily urticarial activity scores (UAS) were derived from weal numbers and itch, maximum 7. There was no significant overall diurnal variation of basophil numbers in healthy controls or chronic urticaria patients. Mean (SE) manually counted basophil were higher in healthy controls than chronic urticaria (43.4/ microL (2.1) vs. 4.4 (0.8), P < 0.001). Basophil counts were reduced in healthy controls on steroids (19.2 (1.9), P < 0.001) but increased in chronic urticaria (8.9 (1.9), P < 0.001). Loratadine did not influence them. UAS fell on treatment (3.3 (0.4) baseline, 1.4 (0.5) on loratadine and 0.5 (0.2) on prednisolone with loratadine, P < 0.001). There was a negative linear correlation between basophil numbers and UAS in untreated chronic urticaria patients (P = 0.001, Spearman rank correlation). Manual and automated basophil counts showed poor agreement. Lymphocyte numbers were lower in chronic urticaria than healthy controls. Neutrophils increased whereas lymphocytes and eosinophils decreased in all subjects on prednisolone. They were unaffected by loratadine. The results are consistent with the hypothesis that circulating basophils may be recruited from blood into urticarial weals during disease activity. Automated counts are not suitable for assessing basophil numbers in chronic urticaria. The relevance of reduced lymphocyte numbers in chronic urticaria needs to be explored.

Chronic urticaria

Chronic urticaria has a spectrum of clinical presentations and causes. About 50% of patients with “idiopathic” disease have histamine-releasing autoantibodies in their blood. The term autoimmune urticaria is increasingly being accepted for this subgroup of patients, in whom immunosuppressive therapies may be appropriate if conventional approaches to management are unsuccessful. This article reviews the classification, causes, and management of chronic urticaria in light of recent advances in the understanding of its etiology. (J Am Acad Dermatol 2002;46:645-57.) Learning objective: At the conclusion of this learning activity, participants should have up-to-date knowledge of the classification, assessment, and management of chronic urticaria and understand where the concept of autoimmune urticaria fits into existing frameworks.

Chronic urticaria serum induces histamine release, leukotriene production, and basophil CD63 surface expression—Inhibitory effects of anti-inflammatory drugs

Background: A role of potential histamine-releasing autoantibodies against the high-affinity IgE receptor on the surface of basophils and mast cells is discussed in the pathogenesis of chronic urticaria. This so-called autoimmune urticaria may be diagnosed by a positive intracutaneous autologous serum skin test, which is found in about 30% of patients with chronic urticaria. Objective: Our purpose was, first, to compare the effect of complement-inactivated sera of 20 patients with chronic urticaria and positive autologous serum skin tests, 20 patients with chronic urticaria and negative skin tests, and 20 control subjects without chronic urticaria (10 atopic and 10 nonatopic subjects) and, second, to analyze the effect of anti-inflammatory drugs on the serum activity. Methods: The following assay systems were used: release of histamine in whole blood samples, surface expression of the activation marker CD63 on basophils, and sulfidoleukotriene de novo production in leukocyte suspensions. Whole blood, basophils, and leukocyte suspensions were obtained from a nonatopic and an atopic donor. Results: Sera of patients with autologous serum skin test positive chronic urticaria resulted not only in significantly increased histamine release compared with skin test–negative chronic urticaria sera but also in a significant higher induction of basophil CD63 surface expression and sulfidoleukotriene de novo production. However, serum activity was neither characteristic for chronic urticaria nor for chronic urticaria with a positive autologous serum skin test. Preincubation with dapsone, chloroquine, and lidocaine dose dependently resulted in a significant reduction of all histamine release, CD63 expression, and sulfidoleukotriene production. In addition, mizolastine was able to inhibit serum-induced sulfidoleukotriene production. Conclusion: Further studies investigating the in vivo effect of these drugs will have to clarify their role in the management of the subset of patients with chronic urticaria demonstrating serum-induced inflammatory effects. (J Allergy Clin Immunol 2000;105:552-60.)

The Pathogenesis of Chronic Idiopathic Urticaria

Chronic idiopathic urticaria (CIU) can be extremely disabling and difficult to treat, with little response to antihistamine therapy. The pathogenic mechanisms of the disease are not well understood, but the primary effector cell is the mast cell. Release of mast cell mediators can cause inflammation and accumulation and activation of other cells, including eosinophils, neutrophils, and possibly basophils. Recent work has demonstrated that about one third of patients with CIU have circulating functional histamine-releasing autoantibodies that bind to the high-affinity IgE receptor (Fc epsilon RI) or, less commonly, to IgE; mast cell-specific histamine-releasing activity that has not yet been fully characterized; no identifiable circulating histamine-releasing activity. The mainstay of treatment of CIU consists of antithistamines, but immunotherapy using plasmapheresis, intravenous immunoglobulin, and cyclosporin may be valuable in severely affected patients with treatment-resistant disease. The response to immunomodulation and the recent finding of an association with HLA DR4 lend further support for an autoimmune basis to CIU in some patients.

Plasmapheresis for severe, unremitting, chronic urticaria

Histamine-releasing autoantibodies have been identified in chronic idiopathic urticaria. 8 patients with severe disease and histamine-releasing activity in their sera underwent plasmapheresis. Symptoms were abolished for 2 months in 1 patient and for 3 weeks in another, 2 showed almost complete resolution of symptoms, 2 had temporary relief, and the other 2 showed little change. Further investigation in 4 of the patients showed significantly reduced skin-test responses to fresh post-exchange autologous sera after plasmapheresis compared with stored pre-exchange sera, but the response to intradermal histamine remained unchanged. Blood cellular histamine increased as in-vitro serum histamine-releasing activity fell after plasmapheresis. These results favour a pathogenetic role for histamine-releasing autoantibodies in patients with chronic urticaria. Lancet 1992; 339: 1078-80

Detection of circulating histamine releasing autoantibodies with functional properties of anti‐IgE in chronic urticaria

Circulating histamine releasing factor(s) have been demonstrated previously in chronic urticaria by an immediate weal-and-flare response to intradermal autologous serum injection. We have studied 25 chronic urticaria patients by in vivo skin testing with autologous sera and an in vitro histamine release assay using mixed leukocytes of healthy donors, to define the nature and functional properties of the serum factor(s). Twenty showed a weal response to autologous serum (mean +/- s.e.m., 37.3 +/- 6.8 mm3). Weal formation was confined to ultrafiltered serum fractions greater than 100 kD in nine of nine patients. There was no response in 10 healthy controls of five patients with symptomatic dermographism. Fourteen chronic urticaria sera elicited histamine release greater than 10% (mean +/- s.e.m., 44.3% +/- 6.7) above basal levels from leukocytes of at least one of seven healthy donors. This in vitro response was also confined to ultrafiltered serum fractions greater than 100 kD in seven of seven sera and was present in IgG fractions of six of seven chronic urticaria sera that showed histamine releasing activity. Functional studies indicated that this histamine releasing autoantibody had the properties of anti-IgE: chronic urticaria sera 'desensitized' basophil leukocytes to subsequent challenge with other chronic urticaria sera and to goat anti-human IgE antibody; human myeloma IgE inhibited histamine release from leukocytes in response to chronic urticaria sera; removal of surface-bound IgE by lactic acid 'stripping' reduced histamine release in response to chronic urticaria sera and anti-IgE and subsequent passive sensitization with IgE myeloma serum partially restored it. Stainable peripheral blood basophils/mm3 in chronic urticaria patients were significantly reduced (mean +/- s.e.m, 7.9 +/- 2.0) when compared to healthy controls (39.6 +/- 4.4), P less than 0.001. These results suggest that histamine releasing autoantibodies are important in the pathogenesis of chronic urticaria by stimulating or facilitating degranulation of basophils and cutaneous mast cells through cross-linking cell surface IgE receptors.

Histamine-Releasing Autoantibodies in Chronic Urticaria

Circulating histamine-releasing factors have been identified in the serum and plasma of chronic-urticaria patients by in vivo skin testing and in vitro histamine release from heterologous mixed leukocytes. Quantitative mast cell studies of serum skin test biopsies and electron microscopy indicate that the serum factors release histamine by mast cell degranulation. Peripheral blood basophils and total cellular blood histamine are reduced in chronic-urticaria patients suggesting that the circulating serum factors cause sustained degranulation. Histamine-releasing activity has been identified by skin testing in ultrafiltered serum fractions less than 30 kDa and greater than 100 kDa. In vitro histamine-releasing activity was confined to ultrafiltered serum fractions greater than 100 kDa and was present in IgG purified from some chronic-urticaria sera by protein G affinity chromatography. The dose-response relationship and kinetics of histamine release in vitro were similar to those of anti-human IgE. 'Desensitisation' of basophils by prior incubation with anti-IgE in the absence of calcium and competitive inhibition studies with myeloma IgE serum indicated that histamine-releasing autoantibodies in chronic-urticaria sera and purified IgG have the properties of anti-IgE. Plasma exchange in 4 patients with active chronic urticaria refractory to antihistamine therapy showing in vivo and in vitro histamine-releasing activity was followed by temporary remission of disease activity in 2 of them. It is possible that chronic urticaria is an autoimmune disease.