Cell-killing mechanisms are essential components of host defense against infectious agents, parasites, and malignant cells. We distinguish humoral and cellular killing mechanisms. Complement utilizes specific proteins to assemble its killer molecule. Macrophages generate active oxygen radicals that injure target membranes. Lymphocytes, like complement, use protein molecules as tools in the cellular cytotoxicity reaction. As will be suggested at the conclusion of this chapter, considerable similarity between the structure of the cytolytic apparatus of complement and that of lymphocytes has recently come to light. An in-depth knowledge of the structure and function of the killer molecule of complement may therefore facilitate elucidation of the cell-killing mechanism of lymphocytes. Complement encompasses 20 proteins, not including the various cell­ surface complement receptors and regulatory proteins. Only 5 of these 20 proteins participate directly in cell killing-C5, C6, C7, C8, and C9-and none has enzymatic activity, proteolytic or lipolytic. Upon activation of C5, the 5 proteins interact in a sequential manner and fuse into a macromo­ lecular organization, called the membrarie attack complex or MAC. Fusion brings forth hydrophobic sites through which the complex inserts itself into the hydrocarbon core of lipid membranes. There it forms transmembrane channels, the largest of which constitutes tubular poly C9. Activation of C5 is accomplished by a highly specific serine protease, C5 convertase, itself an assembly of three protein molecules. Thus, formation of the MAC is initiated enzymatically, but this enzyme does not participate in actual membrane attack, which is entirely a physicochemical process.

The membrane attack complex of complement.

That mast cells and allergic mechanisms play at least some role in bronchial asthma has been suggested for at least 75 years [92]. The incidence of these mechanisms in a cross section of all asthmatics [31, 77], and the importance of allergic mechanisms as compared with neural mechanisms is controversial, however, and has been discussed and debated for at least as long [16, 113].

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Mast cells and basophils.

Large scale isolation of functionally active components of the human complement system.

NOCTURNAL and early morning wheezing is very common in asthmatics, 1–3 but the mechanisms are far from clear. The clinical importance of this symptom is underlined by the fact that many sudden deat...

Nocturnal asthma and changes in circulating epinephrine, histamine, and cortisol.

Urticaria and angioedema are common disorders affecting approximately 15-20% of the population at least once during their lifetime (Swinny, 1941; Sheldon et al., 1954; McKee, 1966). Urticarial lesions result from localized dilatation and transudation of fluid from small venules, and in their simplest form are thought to represent the same sort of wheal-and-flare reaction observed when histamine is injected into the skin. Clinically, one observes the presence of pruritic, erythematous cutaneous wheals that blanch with pressure, reflecting the underlying edema and dilated blood vessels. The histopathological features of urticaria are dilatation of cutaneous blood vessels and lymphatics in the superficial dermis, widening of dermal papillae, flattening of rete pegs, and swelling of collagen fibers.

http://www.fitzpatricksdermatology.com/references/Fitzpatricks_Derm8e_Ref_Ch038.pdf

Urticaria and Angioedema

The molecular arrangement of the membrane attack mechanism of complement was explored. The molar ratios of the components within the C5-9 assembly on the target cell surface were determined using human complement proteins in highly purified and radiolabeled form. With the aid of monospecific complement antisera it was possible to probe the spatial relationships between the components of the assembly. C5 and C6, in the presence of C7, were bound to EAC1-3 in equimolar quantities irrespective of the amounts and the relative proportions of C5, C6, and C7 offered. The amount of C8 bound to EAC1-7 increased with input and at saturation of all C8 binding sites the molar ratio of bound C8/bound C5 approached 1.0. Uptake of C9 by EAC1-8 increased with input and at saturation of all C9 binding sites the molar ratio of bound C9/bound C8 became 6.0. However, calculations suggest that the binding of three C9 molecules to one C8 molecule is sufficient to achieve a full hemolytic effect. Evidence was obtained indicating that binding and hemolytic function of C9 depends upon cooperative interaction of multiple C9 molecules. Binding of C8 to EAC1-7 and the generation of hemolytic C8 sites were inhibited by antibody to either C5, C6, or C7. Uptake of C9 by EAC1-8 and the generation of hemolytic C9 sites were strongly inhibited by anti-C8 and to a lesser degree by anti-C5. Binding of C9 (but not hemolysis) was also reduced by antibody to C6 or C7. The data are consistent with the concept that the fully assembled membrane attack mechanism of complement consists of a decamolecular complex: a trimolecular arrangement composed of C5, C6, and C7 forms the binding site for one C8 molecule which in turn furnishes binding sites for six C9 molecules, saturation of three sites apparently being sufficient for expression of full cytolytic activity of the complex. This work made it possible to design a simple molecular model.

/pdf/molecular-analysis-of-the-membrane-attack-mechanism-of-4rmhpsd1ni.pdf

Molecular analysis of the membrane attack mechanism of complement

Molecular analysis of the m embrane attack mechanism of complement

Seven patients with bronchial asthma underwent bronchial inhalation challenge with aerosolized allergen extracts and methacholine. Simultaneously, venous blood samples were collected and histamine was measured. Each patient was challenged on successive days with an allergen extract to which he had no skin-sensitizing antibody (skin test-negative allergen), followed by methacholine and skin test-positive allergen. Bronchospasm was not induced by inhalation of skin test-negative allergens but was observed in all patients after methacholine and in the majority of patients after skin test-positive allergens. No changes in plasma histamine were detected after challenges with methacholine and skin test-negative allergens. After challenge with skin test-positive allergens, significant rises in plasma histamine were detected in 5 of 7 patients. Plasma histamine was elevated within the first 5 min after inhalation of aerosolized allergen, and elevations persisted as long as 30 min. These studies showing that histamine increases significantly in the plasma during allergen-induced asthma in man suggest that histamine should be considered as at least one of the mediators of bronchospasm in allergic asthma. Bronchospasm induced by the cholinergic drug methacholine, unlike allergen-induced bronchospasm, is not associated with changes in plasma histamine.

https://www.jacionline.org/article/0091-6749(76)90176-7/pdf

Plasma histamine changes during provoked bronchospasm in asthmatic patients.

Venous plasma histamine levels were measured in 60 asthmatic subjects in order to investigate the role of histamine in naturally occurring asthmatic episodes. The patients were divided into 3 groups of 20 according to the degree of disease activity. Group A consisted of asthmatic patients with severe exacerbations. Flow/volume curves demonstrated a forced expiratory volume in 1 sec to forced vital capacity ratio (FEV1FVC) of less than 55% and a maximum expiratory flow at 50% of FVC (MEF 50 ) of less than 1.0 L/sec. Asthmatics in group B were in partial remission. The patients in this group had FEV1FVC ratios between 55% and 75% and MEF 50 values between 1.0 and 4.0 L/sec. The remaining asthmatic patients (group C) were in complete remission. They were asymptomatic and had normal physical examinations and flow/volume curves. Venous plasma histamine levels were also measured in 50 normal control subjects (group D). Every asthmatic subject in group A had plasma histamine above 1.25 ng/ml (mean, 1.9 ng/ml ± 0.5). Only 3 asthmatic patients in group B and 3 in group C had venous plasma histamine levels greater than 1 ng/ml. None of the normal subjects in group D had plasma histamine above 1 ng/ml. These data show a relationship between spontaneously occurring asthmatic attacks and elevated plasma histamine and support previous studies suggesting that histamine may play a role in the mediation of bronchial asthma.

https://www.jacionline.org/article/0091-6749(77)90111-7/pdf

The relationship of plasma histamine to the activity of bronchial asthma

To further an understanding of the effects of radiographic contrast media (RCM) infusions, 43 patients underwent clinical evaluation, including allergy history and skin testing, at least 48 hr before receiving a bolus of Renografin-60 (meglumine diatrizoate [52] and sodium diatrizoate [8]) for intravenous pyelography. Venous plasma samples were obtained serially before and at 2, 4, 10, 30, and 60 min after the infusion. Each sample was assayed for histamine by isotopic enzyme assay, for complement by total hemolytic activity, and for fibrin split products by radioimmunoassay. Immediate generalized reactions occurred in 6 of the patients. Each of the 43 patients had a change in at least 1 of the mediators measured. Plasma histamine rose in 40% of the patients (mean, 3.1 ± 1.62 ng/ml). Plasma complement hemolytic activity decreased in 63% (mean, 29 ± 19%). Fibrin split products were detected in 41%. In comparing the 6 patients who experienced reactions to the 37 who did not, there were no statistically significant differences in rise in plasma histamine, decrease in complement activity, presence of fibrin split products, personal history of allergy, prior exposure or reaction to RCM, skin tests to RCM, and histamine or saline. Although histamine levels, complement activity, and fibrin split products changed in a substantial number of patients undergoing RCM infusions, these changes did not correlate with immediate generalized reactions or with the previously described clinical parameters. It is probable that other modulating factors play significant roles in determining whether or not a reaction occurs.

https://www.jacionline.org/article/0091-6749(79)90114-3/pdf

Carlos M. Arroyave

Papers

Molecular analysis of the membrane attack mechanism of complement

Molecular analysis of the m embrane attack mechanism of complement

Plasma histamine changes during provoked bronchospasm in asthmatic patients.

The relationship of plasma histamine to the activity of bronchial asthma

Radiographic contrast media infusions: Measurement of histamine, complement, and fibrin split products and correlation with clinical parameters☆