Allergy and allergic disease

Question 1

How many types of allergic response are there?

Answer 1

4

Question 2

What is a type 1 allergic response?

Answer 2

IgE is formed as a result of prior sensitization (i.e., contact with the antigen) and coats mast cells and basophils.
Subsequent encounter with antigen results in an IgE-mediated reaction by preformed IgE antibodies: Free antigen binds to two adjacent IgE antibodies (crosslinking) → degranulation of cells
Release of histamine and other mediators (e.g., prostaglandin, platelet-activating factor, leukotrienes, heparin, tryptase) →
Increased smooth muscle contraction + peripheral vasodilation + increased vascular permeability (→ bronchospasm, abdominal cramping and rhinitis) → hypovolemia, hypoxia
Extravasation of capillary blood (→erythema)
Fluid shift into the interstitial space (→ edema, pulmonary edema)
Pruritus

Question 3

What is a type 2 allergic response?

Answer 3

IgM and IgG bind to antigens on cells in the body mistakenly detected as foreign.
Complement activation
Cellular lysis or phagocytosis
Opsonization → phagocytosis or complement activation
Complement-mediated lysis
Antibody-dependent cell-mediated cytotoxicity (NK cells or macrophages)
Type II is cy-2-toxic.

Question 4

What is a type 3 allergic response?

Answer 4

Antigen (e.g., the molecules of a drug in circulation) binds to IgG to form an immune complex = antigen-antibody complex
Immune complexes are deposited in tissue, especially blood vessels → initiation of complement cascade → release of lysosomal enzymes from neutrophils → cell death → inflammation → vasculitis

Question 5

What is a type 4 allergic response?

Answer 5

T cell-mediated reaction
Sensitization: Antigen penetrates the skin → uptake by Langerhans cell → migration to lymph nodes and formation of sensitized T lymphocytes
Eruption: Repeated contact with antigen → secretion of lymphokines and cytokines (e.g., IFNγ, TNFα) by presensitized T lymphocytes → macrophage activation and inflammatory reaction in tissue

Question 6

What is the acronym ACID with regards to hypersensitivity reactions?

Answer 6

The hypersensitivity reactions can be memorized with the mnemonic ACID: A – Allergic/Anaphylactic/Atopic (Type I); C – Cytotoxic (Type II); I – Immune complex deposition (Type III); D – Delayed (Type IV)

Question 7

What is scomboid poisoning?

Answer 7

Foodborne illness caused by ingestion of histidine-rich fish that has spoiled or been inadequately refrigerated. Certain types of fish, including tuna, mackerel, mahi-mahi, and bonito, contain high levels of histidine, which is converted to histamine by bacteria at warm temperatures. Because histamine is also involved in allergic reactions, scombroid is sometimes confused with an allergic reaction to fish. Typically presents with flushing, urticaria, nausea, edema, diarrhea, and abdominal cramps 5 minutes to 2 hours after ingestion. Symptoms usually resolve within a few hours.