Immunology: Allergic Reactions Quiz

Allergy and Immune Response to Parasites

• Immunology course, BIO 371
• Instructor: Dr. Amro Soliman
• Contact information provided

Allergens

• Allergens are harmless substances that trigger allergic reactions in sensitive individuals
• Examples of allergens include:
  • Inhaled: pollen, dust mites
  • Injected: insect venom, drugs
  • Ingested: peanuts, shellfish
  • Contacted: plant oil, metal

Immune Responses to Allergens and Parasites

• Allergic and parasitic infections share similar immune responses
• The main antibody involved in allergic reactions and parasitic infections is IgE
• IgE does not directly bind to antigens (Ag) alone
• IgE binds to Fc receptors on mast cells, basophils, and eosinophils
• IgE bound to these cells recognizes and binds to antigens
• This triggers the release of inflammatory mediators, such as histamine
• This promotes inflammation and helps kill parasites

Types of Hypersensitivity Reactions

• Classified according to effector mechanisms:
  • Type I: Immediate hypersensitivity (e.g., contact allergies, asthma)
  • Type II: Antibody-mediated hypersensitivity (e.g., drug reactions)
  • Type III: Immune complex-mediated hypersensitivity (e.g., serum sickness)
  • Type IV: Cell-mediated hypersensitivity (e.g., poison ivy reaction)

Type I Hypersensitivity

• Primary adaptive immune response generates IgE specific to the allergen
• IgE circulates and binds to Fc receptors on mast cells, basophils, and eosinophils
• Subsequent exposure triggers degranulation and release of inflammatory mediators (e.g., histamine)
• This leads to immediate hypersensitivity reactions

Type II Hypersensitivity

• Immune response to chemically reactive small molecules (e.g., drugs)
• Modifies cell surface components (self-antigens), making them foreign
• B cells produce IgG against new epitopes
• IgG triggers complement activation and opsonization
• Leading to inflammation and tissue damage

Type III Hypersensitivity

• Caused by therapeutic antibodies from animals (e.g., mice)
• Animal antibodies are recognized as foreign antigens (Ag)
• Human antibodies bind to animal antibodies forming immune complexes
• These complexes deposit in blood vessels and alveoli, activating complement system
• Leading to inflammation and severe disease (serum sickness)

Type IV Hypersensitivity

• Allergic immune response to metals (e.g., nickel)
• Modified peptides are recognized as foreign antigens by dendritic cells (DCs) and T cells
• Activates CD4 T cells, triggering an adaptive immune response
• Leading to inflammation and tissue damage, including redness, itchiness, and blistering
• Delayed-type hypersensitivity (DTH) as it takes 24-72 hours to develop

IgE-Mediated Allergic Reactions

• Consist of an immediate response followed by a late-phase response, for instance, in contact allergic reactions.
• Immediate: Within minutes following exposure (e.g., wheal-and-flare reactions mediated by IgE).
• Late-phase: 6–8 hours after exposure (more swelling induced by cytokines, chemokines, and inflammatory cells lasting for a longer period).

Physical Effects of IgE-Mediated Mast-Cell Degranulation

• Vary based on the tissue exposed to allergens.
• Gastrointestinal tract: increased fluid secretion, expulsion of tract contents.
• Airways: decreased diameter, mucus secretion, coughing, sneezing
• Blood vessels: increased blood flow, increased permeability, edema, inflammation
• Skin: swelling, redness
• Everywhere: Anaphylaxis symptoms  

Allergic Diseases of the Respiratory Tract

• Nasal cavity: allergic rhinitis
• Lower respiratory airways: asthma.
• Diagrams of respiratory system are present

Sensitization to Inhaled Allergens

• Airborne particles get caught in mucus and are broken down
• Antigen peptides are presented by antigen-presenting cells (APCs) to T cells
• This stimulates Th2 response, leading to IgE production
• IgE binds to mast cells

Allergic Rhinitis

• Local edema and nasal discharge are observed
• Discharge is rich in eosinophils
• Associated with irritation, itchiness, and sneezing

Allergic Asthma: Acute Response

• Activation of mucosal mast cells (upon allergen exposure), leads to severe effects
• Including shortness of breath and wheezing
• Mediators from mast cells cause infiltration of Th cells, neutrophils and mast cells
• Resulting in further inflammation within lung tissues

Allergic Asthma: Chronic Response

• Frequent allergen exposure leads to chronic inflammation
• Continuous release of cytokines instigates further inflammation (a vicious cycle)
• Airways are blocked by mucus for a prolonged period

Skin

• Structure of skin layers is visualized

Wheal-and-Flare Reaction

• Histamine dilates blood vessels, causing fluid and protein leakage (swelling), and redness.

Skin Allergic Reactions

• Urticaria/hives (superficial)
• Angioedema (subcutaneous tissue edema)

Gastrointestinal Tract

• Diagram of digestive system is available

Ingestion of Food/Drugs

• Certain foods and drugs (e.g., peanuts, penicillin, aspirin) can cause allergies
• Allergens trigger intestinal smooth muscle contraction, causing local symptoms
• Systemic symptoms (urticaria) can occur if allergens diffuse into the bloodstream.

Anaphylaxis

• Severe systemic allergic reaction
• Mediated by mast cell activation throughout the body
• Leads to various tissue-specific effects, resulting in shock and/or respiratory compromise
• Death can occur

Prevention and Treatment

• Avoid allergens
• Drugs (antihistamines, steroids)
• Desensitization therapy (allergy shots)

Parasites

• Organisms that live on or inside the host.
• Cause chronic diseases by competing with the host for nutrients and inducing tissue damage
• Classified into three major groups:
  • Protozoa (single-celled organisms), e.g., Plasmodium, Giardia
  • Helminths (worm-like multicellular parasites), e.g., nematodes, trematodes, cestodes
  • Ectoparasites (live on the surface of the host), e.g., fleas, lice, ticks, mites

Worldwide Distribution of Parasitic and Allergic Diseases

• Maps showing global distribution

Tenets of the Hygiene Hypothesis

• Excessive hygiene reduces childhood exposure to microorganisms
• This reduces immune system development and exposure to real-world illnesses, leading to allergies

Immune Response to Parasites

• B cells recognize parasitic antigens, process them and present to Th2 cells.
• Th2 cells activate B cells and release cytokines, leading to B cell activation, somatic hypermutation, and class switching to IgE.
• This results in the production of IgE-secreting plasma cells, providing a humoral response.
• IgE also targets parasites

How IgE Fight Parasitic Infections

• IgE does not bind to antigens alone.
• IgE binds to Fc receptors on mast cells, basophils, and eosinophils.
• IgE bound to Fc receptors binds to parasite antigens, stimulating degranulation (release of granules).
• Granules release mediators (e.g., histamine, proteases) killing parasites and causing local inflammation and collateral tissue damage
• Inducing muscle spasms and diarrhea to eliminate parasites.

IL-5, a Major Cytokine in Parasitic Infections

• Th2 cells secrete IL-5
• Crucial for eosinophil development and function
• Increase eosinophils in blood and infected tissues
• Eosinophils express Fc receptors to bind to IgE associated with parasitic antigens and stimulate degranulation to kill the parasites

Mediators Produced by Eosinophils

• Eosinophils produce various molecules including enzymes, toxins, and mediators
• These molecules kill parasites, induce histamine release, remodel connective tissue, and increase vascular permeability.

Mast Cells in Allergic and Anti-Parasitic Responses

• Mast cells display a high number of Fc receptors armed with different IgE specificities.
• Can also be armed with IgE specific to parasites (blood fluke, hookworm, roundworm) and allergens (pollen, peanuts, cat dander, bee venom)

Basophils

• Similar function as mast cells, and express Fc receptors for binding IgE and IgD
• Recognize specific antigens (allergens or parasites)
• Mast cells, eosinophils, and basophils often work together.