ABAI cheat sheet
ABAI · Keep the key ideas within reach.
Allergy
Type I hypersensitivity is IgE-mediated: allergen cross-links mast-cell FcεRI-bound IgE, triggering degranulation within minutes.
Type IV reactions are T-cell mediated and delayed 48–72 hours; patch testing reads at 48 and 72–96 hours.
Anaphylaxis criteria: acute onset with skin/mucosal involvement plus respiratory compromise or hypotension, or hypotension after known allergen exposure.
Intramuscular epinephrine 0.01 mg/kg of 1:1000 (max 0.5 mg) into the anterolateral thigh is first-line for anaphylaxis.
Serum tryptase peaks 1–2 hours after anaphylaxis onset; a elevated acute level supports the diagnosis retrospectively.
Allergic rhinitis is IgE-mediated; intranasal corticosteroids are the most effective single maintenance therapy for moderate-severe disease.
Skin prick testing has high sensitivity but low specificity; specific IgE serum testing is preferred when skin disease or antihistamines preclude prick testing.
Asthma is a chronic airway inflammation with reversible obstruction; spirometry showing ≥12% and ≥200 mL FEV1 improvement after bronchodilator supports diagnosis.
Drug allergy: beta-lactam skin testing followed by graded challenge is standard for suspected immediate penicillin reactions when history is low-risk.
Immunology
IgG is the most abundant serum immunoglobulin and the only class that crosses the placenta (via FcRn).
IgM is the first antibody produced in a primary response and is a pentamer; it is the most efficient at complement activation.
IgA exists as monomeric serum and dimeric secretory IgA (with J chain and secretory component) at mucosal surfaces.
IgE binds FcεRI on mast cells and basophils; its serum level is the lowest of all immunoglobulin classes.
Complement classical pathway: C1q binds IgG/IgM → C4 → C2 → C3 convertase (C4b2a) → C3b → C5 convertase → membrane attack complex C5b-9.
T cells: CD4+ recognize antigen on MHC class II; CD8+ recognize antigen on MHC class I.
Primary immunodeficiencies: X-linked agammaglobulinemia (BTK defect) causes absent B cells and low all immunoglobulins; SCID presents in infancy with failure to thrive and infections.
Hypersensitivity classification: I (IgE), II (antibody-mediated cytotoxic), III (immune complex), IV (delayed T-cell).
CBC with differential and quantitative immunoglobulins are first-tier tests for suspected primary immunodeficiency.