Immunology and asthma

Question 1

Describe asthma

Answer 1

Chronic disease of the airways
Characterized by
- Recurrent episodes of airway obstruction
- Bronchial hyper-responsiveness
- Inflammation of the airways
- Changes in function and structure of airways only partially reversible

• Heterogeneous disease
• Move to classify according to phenotype
• Pattern of airway inflammation, potential triggers, associated diseases
• Most mild-moderate asthma associated with atopy
• Refractory asthma - subphenotypes
• Main aim of subcategorization to optimize treatment regimens

Question 2

Describe contributory factors to asthma

Answer 2

• Genetic
• Many genetic variants contribute to disease risk through interaction with a range of environmental factors: Increased risk of asthma with particular environment exposure - LTC4s, lipo-oxygenase (aspirin sensitivity)
  • Increased bronchial hypersensitivity
  • Lowered FEV1
  • Decreased risk of asthma - TLR4, endotoxin exposure,
  carriers of particular alleles have reduced risk of asthma
• Diet, allergen and microbial exposure, exact role and contribution remains uncertain
• Atopy
  - Familial predisposition to the development of IgE mediated responses to common environmental antigens
  - 10-20 fold increase in risk of developing asthma
  - Severe asthma
• History of aspirin sensitivity
• Intercurrent GORD
• Increased risk of sinusitis, pneumonia
• Females

Question 3

List some modifying environmental factors

Answer 3

• Infections
• Birth during periods of high pollen counts
• No of siblings (first)
• Early attendance at day care centers
• Early exposure to animals
• Exposure to aeroallergens - cockroach, house dust mite

Question 4

Describe genetic factors

Answer 4

• No genes associated with asthma, atopy or a related phenotype are consistently identified in reported studies
• rather genetic influence is more of an interplay between genes and environment
• presence alone does not increase or decrease risk
• Genetic polymorphisms associated with an
• Increased risk of asthma with particular environmental exposure
• LTC4S, 5 lipo-oxygenase - aspirin sensitivity
• Decreased risk of asthma
• TLR4 - high levels of endotoxin exposure - carriers of particular alleles have a reduced risk of asthma
• ADAM 33 (metalloproteinase) - increased risk of asthma in a subgroup of patients only
• β2 adrenergic receptor - increased bronchial hyperreactivity
• IL-4, TGF β1
• IL4-Rα - Severe exacerbations (including ICU admission), lower FEV1
• 17q21 variants associated with early-onset asthma (with exposure to cigarette smoke) without risk of eczema, rhinitis/allergic sensitization
• Filaggrin – early onset eczema followed by acute severe asthmatic exacerbations and later sensitization

Question 5

Describe environmental triggers

Answer 5

• Exacerbate rather than cause allergic disease; not to be confused with environmental factors
• Atmospheric pollutants (indoor & external)
• Cigarette smoke
• Fossil fuel combustion
• Volatile organic compounds
• Viral infections esp Respiratory syncytial virus

Question 6

Describe the process of sensitisation

Answer 6

• Allergen contact with mucosal surfaces either by Inhalation, Ingestion or Injection
• Many allergens have enzymatic activity and thus disrupt epithelial junctions enhancing antigen uptake
• Size of particle important** as well Aerodynamic properties - effective dispersion and Deposition in airways

Tolerance occurs if barrier function is intact (due to action of mucociliary escalator and epithelial tight junctions). Tolernace is maintained by immature DCs, Tregs and destruction of antigen specific T cell populations.

Response occurs if barrier is disrupted, DCs mature, activate and increase, Th2 cells produces cytokines which prime the environment for allergy.

Question 7

Describe the acute response

Answer 7

• In sensitized individuals
• Exposure of the airways to specific allergens results in crosslinking of IgE present on the surface of mast cells and triggers the release of inflammatory mediators (see mast cell role question).

Note: seen antigen before. Mast cells recognise via receptor interactions, cause aggregation of antigens, triggering degranulation.

• Increased vascular permeability
• Bronchial smooth muscle contraction
• Increased mucous secretion.
• Allergic airway phenotype results from cytokine production from T cells as well as from inflammatory mediators released from recruited eosinophils and other cells in the lung.
• Mucous hypersecretion
• Smooth muscle cell hyperreactivity
• Airway remodeling with chronic inflammation.

Question 8

Describe asthma as an inflammatory disease/describe the structural changes

Answer 8

-
- Infiltration by neutrophils and eosinophils
- Mucous plugging of the lumen
-
-

Structural changes of airway wall
• Cellular and molecular components of bronchial wall
• Epithelial injury
• Sub epithelial thickening/fibrosis
• Airway smooth muscle hyperplasia (size and
numbers)
• Goblet hypertrophy and hyperplasia
• Angiogenesis (formation of new blood vessels)
• Remodelling occurs in parallel with inflammation • Smooth muscle hypertrophy and hyperplasia
• Large and small airways
• Increased propensity to recruit and retain inflammatory
cells including mast cells, neutrophils and monocytes (neutro and eosino infiltration)
• Elevated levels IL-13, TNF and local environment

Question 9

Describe the chronic response in asthma

Answer 9

• Influx of inflammatory cells
• Th2 cells, eosinophils
• Cytokines produced by both eosinophils and T cells promote the homing of eosinophils to the site of inflammation followed by differentiation, activation and degranulation
• Cycle of ongoing tissue injury
• Increase in inflammatory infiltrate
• Chronic inflammation - perpetuated by both cytokines and the products released by eosinophils
• Increasing evidence that other subsets of CD4+ T cells play a role in orchestrating inflammatory changes
• Th1, Th17, Treg, CD1d restricted NKT cells, TFH

Question 10

Describe the role of cellular interactions in asthma

Answer 10

• ## CD4+ T cells play a key role
• Expression of activation markers, increased mRNA, production of Th2 cytokines IL-4, IL-5, IL-13
• Increased even during quiescent periods
• Recall Th2 effector roles: B cell switching to IgE, eosino and baso recruitment, mast cell differentiation, airway hyper-reactivity, mucuous hyper secretion, epithelial damage and fibrosis

Animal models - no induction of allergic airway disease in the absence of Th2 cells
- Increased in the airways of asthmatic patients

Question 11

Describe effector functions of Th2

Answer 11

• CD4+ Th2 cell
• B cells - Isotype switch to IgE production (IL-4, IL-13)
• Eosinophil & basophil recruitment (IL4, IL-5, IL-9, IL-13)
• Mast cell differentiation (IL-4, IL-9, IL13)
• Airway hyper-reactivity (IL-9, IL-13)
• Epithelial damage and fibrosis (IL-13)
• Mucous hyper-secretion (IL-4, IL-9, IL-13)

Question 12

Describe effector functions of eosinophils

Answer 12

• CD4+ T cell provides key signals leading to activation and recruitment
• Secretion of proteins, cytokines, chemokines
• ECP, MBP, TNF, GM-CSF, IL-4, -13, -5, eotaxin, PDGF
  
  • Airway damage
  • Mucous secretion
  • Bronchial hyper reactivity
  • Fibrosis

Eosinophils are thought to play a key role in the augmentation of inflammation through the production of cytokines such as IL-13. They also contribute to airway remodeling by promoting subepithelial fibrosis and tissue damage.

Question 13

Describe role of adenosine

Answer 13

• Generated from dephosphorylation of adenine nucleotides released from inflammatory and injured cells.
• Increased in the blood and airways of individuals with asthma.
• Induces bronchospasm.
• May act to increase the production of IL-13, leading to further airway damage.

Question 14

Describe role of MCP-1

Answer 14

• Produced by bronchial epithelial cells, macrophages, and smooth muscle cells.
• Release is triggered by the presence of IL-13 and adenosine.
• It recruits Th2 cells to the airways and differentiates them.
• Possible role in polarizing the response to Th2 cytokines (IL-4, IL-5, IL-13).

Question 15

Describe role of NO

Answer 15

• Generation of nitric oxide is associated with ongoing airway inflammation.
• Exhaled NO correlates with inflammation, type I/type II cytokine imbalance, and TGF-beta levels.
• **Conducting airway NO is associated with
  
  • increased basement membrane thickness
  • increaed expression of matrix metalloproteinases**
• indicating a role in the persistence of airway inflammation and remodeling.
• Experimentally and clinically decreased levels correlate with response to therapy.

Question 16

Describe role of MMPs

Answer 16

• A family of proteinases that regulate the deposition of collagen.
• Act in concert with tissue inhibitors of metalloproteinases (TIMPs).
• Elevated in pulmonary diseases where remodeling is prominent.
• Play a role in extracellular matrix remodeling and leukocyte migration to sites of inflammation.

Question 17

Describe role of TGFb

Answer 17

• A mediator of remodeling.
• Synthesized by airway cells, epithelial cells, fibroblasts, and eosinophils.
• Stimulates fibroblasts to produce ECM proteins and myofibroblasts to produce collagen.
• Expression correlates with subepithelial fibrosis.

Question 18

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Describe pathophysiology of asthma

Answer 18

There are two factors that contribute to the development of asthma.
These include changes to the airway walls and lumen.
The airway walls become infiltrated with mononuclear cells, especially CD4+ T cells and eosinophils.
Severe asthma is associated with increasing neutrophil infiltration. This is because, as asthma progresses, there are increased numbers of degranulated mast cells, macrophages, plasma cells and neutrophils found in the airway walls.

Changes within the airway lumen include increased secretions, containing lymphocytes, activated macrophages, eosinophils and epithelial cells.

With inflammation comes remodelling of the airway.
This involves cellular and molecular changes to the airway, affecting its structure.
Changes include:
- epithelial injury
- subepithelial thickening and fibrosis
- airway smooth muscle cell hyperplasia. This in particular affects recruitment of inflammatory cells.
- goblet hypertrophy and hyperplasia
- angiogenesis

These structural changes contribute to

• bronchial hyper-responsiveness which is correlated with inflammation, and is influenced by diameter of lumen, muscle contractility, epithelial injury, neuronal deregulation and microvascular permeability, airway obstruction, and associated symptoms characteristic of asthma including breathlessness, wheezing and coughing.
  - Airway obstruction which is characterised as increased airway smooth muscle mass, increased matrix protein deposition, disruption of surfactant function, and excess mucous with extravasated proteins and inflammatory cells comprising plugs.

Question 19

Describe the consequences of inflammation and airway remodelling

Answer 19

• BHR
• Airway obstruction
  
  • increased ASM mass
  • increase in matrix protein deposition
  • disruption of surfactant function
  • excess mucous, extravasated protein and inflammatory cells, comprising plugs
• Breathlessness, wheeze and cough

BHR
- correlation with degree of infllammtion
- contributory factors include: reduced luminal diameter, smooth muscle contractility, epithelial injury, neuronal dysregulation and microvascular permeability

Question 20

Describe beta 2 adrenergic agonists

Answer 20

• Beta-2 adrenergic agonists
  Relaxation of smooth muscle
  Promotion of mucociliary clearance
  Reduce vascular permeability
  Modulate release of mast cell mediators

Question 21

Describe leukotriene antagonists

Answer 21

Inhibit exercise induced bronchospasm
Modify airway response to allergens
Adjunctive role with inhaled corticosteroids

Question 22

Describe glucocorticoids

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Answer 22

Reduction in inflammatory cell infiltrate
Reduce vascular permeability
Decrease in mucous production
Increase in beta-adrenergic response

Question 23

List various biological therapies

Answer 23

Biologic therapies (e.g., Omalizumab ^[reduces exacerbationsm stabilises lung function, targets IgE], Mepolizumab ^[targets IL5], Resilizumab ^[targets IL-5], Benralizumab ^[IL-5R], Dupilumab ^[IL 4 and 13, redults in 50% reduction in asthma exaverbations, some evidence for improved lung function, patient perception of improvement, reduced systemic corticosteroids and mossibility of remission.reduction in sequlae associated with asthma])

Question 24

List some other therapies

Answer 24

low-dose long-term macrolide therapy, immunomodulatory agents, anti-TNF therapy, tyrosine kinase inhibitors, specific immunotherapy, and bronchial thermoplasty.

Question 25

Describe sensitjsation and activation of mast cells

Answer 25

Sensitization and Activation of Mast Cells
Because mast cells are central to the development of imme- diate hypersensitivity, we first review some of their salient characteristics. Mast cells are bone marrow–derived cells that are widely distributed in the tissues. They are abun- dant near blood vessels and nerves and in subepithelial tissues, which explains why local immediate hypersensi- tivity reactions often occur at these sites. Mast cells have cytoplasmic membrane-bound granules that contain a variety of biologically active mediators, described later. The granules also contain acidic proteoglycans that bind basic dyes such as toluidine blue. (Mast in German refers to fattening of animals, and the name of these cells came from the erroneous belief that their granules fed the tissue where the cells were located.) As is detailed next, mast cells (and their circulating counterpart, basophils) are activated by the cross-linking of high-affinity IgE Fc receptors; in addition, mast cells may also be triggered by several other stimuli, such as complement components C5a and C3a (called anaphylatoxins because they elicit reactions that mimic anaphylaxis), both of which act by binding to receptors on the mast cell membrane. Other mast cell secre- tagogues include some chemokines (e.g., IL-8), drugs such as codeine and morphine, adenosine, melittin (present in bee venom), and physical stimuli (e.g., heat, cold, sunlight). Basophils are similar to mast cells in many respects, includ- ing the presence of cell surface IgE Fc receptors as well as oplasmic granules. In contrast to mast cells, however, basophils are not normally present in tissues but rather circulate in the blood in extremely small numbers. Similar to other granulocytes, basophils can be recruited to inflam- matory sites.

Mast cells and basophils express a high-affinity recep- tor, called FcεRI, which is specific for the Fc portion of IgE and therefore avidly binds IgE antibodies. IgE-coated mast cells are said to be sensitized, because they are sensitive to subsequent encounter with the specific antigen. When a mast cell, armed with IgE antibodies previously pro- duced in response to an antigen, is exposed to the same antigen, the cell is activated, leading eventually to the release of an arsenal of powerful mediators responsible for the clinical features of immediate hypersensitivity reactions. In the first step in the sequence of mast cell acti- vation, the antigen binds to the IgE antibodies previously attached to the mast cells. Multivalent antigens bind to and cross-link adjacent IgE antibodies. The underlying Fcε receptors are brought together, and this activates signal transduction pathways from the cytoplasmic portion of the receptors. These signals lead to the production of media- tors that are responsible for the initial, sometimes explo- sive, symptoms of immediate hypersensitivity, and they also set into motion the events that lead to the late-phase reaction.

Question 26

Describe the mediators of immediate hypersensitivity

Answer 26

Mast cell activation leads to degranulation, with the dis- charge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of second- ary mediators, including lipid products and cytokines (Fig. 6-15).
Preformed Mediators. Mediators contained within mast cell granules are the first to be released and can be divided into three categories:
• Vasoactive amines. The most important mast cell-derived amine is histamine (Chapter 3). Histamine causes intense smooth muscle contraction, increased vascular perme- ability, and increased mucus secretion by nasal, bron- chial, and gastric glands.
• Enzymes. These are contained in the granule matrix and include neutral proteases (chymase, tryptase) and several acid hydrolases. The enzymes cause tissue damage and lead to the generation of kinins and acti- vated components of complement (e.g., C3a) by acting on their precursor proteins.
• Proteoglycans. These include heparin, a well-known anti- coagulant, and chondroitin sulfate. The proteoglycans serve to package and store the amines in the granules.
Lipid Mediators. The major lipid mediators are arachidonic acid–derived products (Chapter 3). Reactions in the mast cell membranes lead to activation of phospholipase A2, an enzyme that converts membrane phospholipids to arachidonic acid. This is the parent compound from which leukotrienes and prostaglandins are produced by the 5-lipoxygenase and cyclooxygenase pathways, respectively.
• Leukotrienes. Leukotrienes C4 and D4 are the most potent vasoactive and spasmogenic agents known. On a molar basis, they are several thousand times more active than histamine in increasing vascular permeability and causing bronchial smooth muscle contraction. Leukotriene B4 is highly chemotactic for neutrophils, eosinophils, and monocytes

Prostaglandin D2. This is the most abundant mediator produced in mast cells by the cyclooxygenase pathway. It causes intense bronchospasm as well as increased mucus secretion.
• Platelet-activating factor (PAF). PAF (Chapter 3) is a lipid mediator produced by some mast cell populations but it is not derived from arachidonic acid. It causes platelet aggregation, release of histamine, broncho- spasm, increased vascular permeability, and vasodila- tion. Its role in immediate hypersensitivity reactions is not well established.
Cytokines. Mast cells are sources of many cytokines, which may play an important role at several stages of immediate hypersensitivity reactions. The cytokines include: TNF, IL-1, and chemokines, which promote leu- kocyte recruitment (typical of the late-phase reaction); IL-4, which amplifies the TH2 response; and numerous others. The inflammatory cells that are recruited by mast cell- derived TNF and chemokines are additional sources of cytokines and of histamine-releasing factors that cause further mast cell degranulation.
These mediators are responsible for the manifesta- tions of immediate hypersensitivity reactions. Some, such as histamine and leukotrienes, are released rapidly from sensitized mast cells and are responsible for the intense immediate reactions characterized by edema, mucus secre- tion, and smooth muscle spasm; others, exemplified by cytokines, including chemokines, set the stage for the late- phase response by recruiting additional leukocytes. Not only do these inflammatory cells release additional waves of mediators (including cytokines), but they also cause epi- thelial cell damage. Epithelial cells themselves are not passive bystanders in this reaction; they can also produce soluble mediators, such as chemokines.

Question 27

Describe the pathogenesis of atopic asthma

Answer 27

Atopic asthma is caused by a TH2 and IgE response to environmental allergens in genetically pre- disposed individuals. Airway inflammation is central to disease pathophysiology and causes airway dysfunction partly through the release of potent inflammatory media- tors and partly through remodeling of the airway wall. As the disease becomes more severe, there is increased local secretion of growth factors, which induce mucus gland hypertrophy, smooth muscle proliferation, angiogenesis, fibrosis and nerve proliferation. Varying combinations of these processes help explain the different asthma subtypes, their response to treatment and their natural history over a person’s lifetime.
The contributions of the immune response, genetics and environment are discussed separately below, although they are closely intertwined.
TH2Responses,IgEandInflammation. Afundamentalabnor- mality in asthma is an exaggerated TH2 response to nor- mally harmless environmental antigens (Fig 15-10). TH2 cells secrete cytokines that promote inflammation and stimulate B cells to produce IgE and other antibodies. These cytokines include IL-4, which stimulates the produc- tion of IgE; IL-5, which activates locally recruited eosino- phils; and IL-13, which stimulates mucus secretion from bronchial submucosal glands and also promotes IgE pro- duction by B cells. The T cells and epithelial cells secrete chemokines that recruit more T cells and eosinophils, thus exacerbating the reaction. As in other allergic reactions (Chapter 6), IgE binds to the Fc receptors on submucosal mast cells, and repeat exposure to the allergen triggers the mast cells to release granule contents and produce cyto- kines and other mediators, which collectively induce the early-phase (immediate hypersensitivity) reaction and the late- phase reaction.
The early reaction is dominated by bronchoconstric- tion, increased mucus production, variable degrees of vasodilation, and increased vascular permeability. Bron- choconstriction is triggered by direct stimulation of sub- epithelial vagal (parasympathetic) receptors through both central and local reflexes triggered by mediators pro- duced by mast cells and other cells in the reaction. The late-phase reaction is dominated by recruitment of leuko- cytes, notably eosinophils, neutrophils, and more T cells. Although TH2 cells are the dominant T cell type involved in the disease, other T cells that contribute to the inflam- mation include TH17 (IL-17 producing) cells, which recruit neutrophils.
Many mediators produced by leukocytes and epithelial cells have been implicated in the asthmatic response. The long list of “suspects” in acute asthma can be ranked based on the clinical efficacy of pharmacologic intervention with antagonists of specific mediators.
• Mediators whose role in bronchospasm is clearly sup- ported by efficacy of pharmacologic intervention are: (1) leukotrienes C4, D4, and E4, which cause prolonged bronchoconstriction as well as increased vascular per- meability and increased mucus secretion, and (2) acetyl- choline, released from intrapulmonary parasympathetic nerves, which can cause airway smooth muscle constric- tion by directly stimulating muscarinic receptors.
• A second group includes agents present at the “scene of the crime” but whose actual role in acute allergic asthma seems relatively minor on the basis of lack of efficacy of potent antagonists or synthesis inhib- itors: (1) histamine, a potent bronchoconstrictor; (2) pros- taglandin D2, which elicits bronchoconstriction and vasodilatation; and (3) platelet-activating factor, which causes aggregation of platelets and release of serotonin from their granules. These mediators might yet prove important in certain types of chronic or nonallergic asthma.
• Finally, a large third group comprises the “suspects” for whom specific antagonists or inhibitors are not avail- able or have been insufficiently studied as yet. Several promising focused therapies for asthma that target the IL-13/IL-4 signal transduction pathways are in develop- ment, including anti-IL-13 monoclonal antibodies and IL-4 receptor antagonists. Other targets include IL-1, TNF, IL-6, chemokines (e.g., eotaxin, also known as CCL11), neuropeptides, nitric oxide, bradykinin, and endothelins

It is thus clear that multiple mediators contribute to the acute asthmatic response. Moreover, the composition of this “mediator soup” might vary among individuals or different types of asthma. The appreciation of the impor- tance of inflammatory cells and mediators in asthma has led to greater emphasis on anti-inflammatory drugs, such as corticosteroids, in its treatment.

Question 28

Distinguish between atopic and other types of asthma

Answer 28

Asthma may be categorized as atopic (evidence of allergen sensitization and immune activation, often in a patient with allergic rhinitis or eczema) or non-atopic (no evidence of allergen sensitization). In either type, episodes of bronchospasm can have diverse triggers, such as respi- ratory infections (especially viral infections), exposure to irritants (e.g., smoke, fumes), cold air, stress, and exercise. There is some evidence that sub-classifying asthma accord- ing to its clinical features and underlying biology is clini- cally useful. One example is early-onset allergic asthma associated with TH2 helper T cell inflammation, a feature seen in about half of the patients. This form of asthma responds well to corticosteroids. However, there is no current consensus as to definitions and diagnostic criteria. Asthma may also be classified according to the insert cintent here.

Atopic c Asthma. This most common type of asthma is a classic example of IgE-mediated (type I) hypersensitivity reaction, discussed in detail in Chapter 6. The disease usually begins in childhood and is triggered by environ- mental allergens, such as dusts, pollens, cockroach or animal dander, and foods, which most frequently act in synergy with other proinflammatory environmental cofac- tors, most notable respiratory viral infections. A positive family history of asthma is common, and a skin test with the offending antigen in these patients results in an imme- diate wheal-and-flare reaction. Atopic asthma may also be diagnosed based on high total serum IgE levels or evidence of allergen sensitization by serum radioallergosorbent tests (called RAST), which can detect the presence of IgE anti- bodies that are specific for individual allergens.
Non-Atopic Asthma. Individuals with non-atopic asthma do not have evidence of allergen sensitization and skin test results are usually negative. A positive family history of asthma is less common in these patients. Respiratory infec- tions due to viruses (e.g., rhinovirus, parainfluenza virus and respiratory syncytial virus) are common triggers in non-atopic asthma. Inhaled air pollutants, such as smoking, sulfur dioxide, ozone, and nitrogen dioxide, may also con- tribute to the chronic airway inflammation and hyperreac- tivity in some cases. As already mentioned, in some instances attacks may be triggered by seemingly innocuous events, such as exposure to cold and even exercise.

Drug-Induced Asthma. Several pharmacologic agents pro- voke asthma. Aspirin-sensitive asthma is an uncommon type, occurring in individuals with recurrent rhinitis and nasal polyps. These individuals are exquisitely sensi- tive to small doses of aspirin as well as other nonsteroidal anti-inflammatory medications, and they experience not only asthmatic attacks but also urticaria. Aspirin (and other non-steroidal anti-inflammatory drugs) triggers asthma in these patients by inhibiting the cyclooxygenase pathway of arachidonic acid metabolism, leading to a rapid decrease in prostaglandin E2. Normally prosta- glandin E2 inhibits enzymes that generate proinflamma- tory mediators such as leukotrienes B4, C4, D4 and E4, which are believed to have central roles in aspirin-induced asthma.
Occupational Asthma. This form of asthma may be trig- gered by fumes (epoxy resins, plastics), organic and chemi- cal dusts (wood, cotton, platinum), gases (toluene), or other chemicals (formaldehyde, penicillin products). Only minute quantities of chemicals are required to induce the attack, which usually occurs after repeated exposure. The underlying mechanisms vary according to stimulus and include type I reactions, direct liberation of bronchocon- strictor substances, and hypersensitivity responses of unknown origin.