The whole effector axis in one chain: Th2 cytokines arm the IgE switch, IgE sits on mast cells, allergen cross-linking fires the gun, and eosinophils finish the late damage. This page owns the mechanism depth.
Opening challenge. A 5-year-old boy is exposed to latex gloves during a dental procedure and, on testing, is found to have produced anti-latex IgM antibodies. He has no reaction at the time. Ten months later he returns for a routine cleaning, and within minutes of latex contact he develops lip swelling, diffuse hives, and wheezing. Serology now shows a high level of serum anti-latex IgE. Which cytokine is most likely responsible for the change in his anti-latex antibody isotype from IgM to IgE?
IL-2
IL-4
IL-5
IL-12
IFN-gamma
Good instinct reaching for a T-cell cytokine, because an isotype switch always needs T-cell help. But IL-2 just makes T cells divide, and IL-5 commands eosinophils, not the IgE switch. IL-12 and IFN-gamma push the Th1 axis, which actually suppresses IgE. The single cytokine that delivers the DNA-recombination signal to switch the B-cell heavy chain to epsilon is IL-4 from a Th2 cell. See "class switch to IgE" in the stem? It is IL-4, every time.
Arm the Response: The Th2 Cytokines
Three Th2 cytokines split the work of building Type I hypersensitivity. Match each job to the cytokine that does it.
1Which cytokine tells the B cell to class-switch to IgE?
Tap a cytokine to commit.
Score: 0 of 0
Pull the Trigger: Cross-Link the Receptor
Sensitizing IgE is already bound to mast cells. A multivalent allergen has to bridge two receptors at once. Find the configuration that actually fires degranulation.
Chapter I · Second Exposure
MAYA
Age 6
Maya was sensitized to peanut months ago. Her mast cells are now coated with anti-peanut IgE. Today she ate a peanut butter cookie. Tap begin to examine her mast cell.
Which configuration triggers release?
Tap the configuration that aggregates the receptors.
Pattern Locked
Receptor Aggregation
One allergen bridging two IgE molecules is the whole signal.
PearlA single IgE bound does nothing. Cross-linking is the trigger.
Run Complete
0 / 0
Receptor aggregation sealed. The mast cell is a loaded gun and cross-linking is the trigger.
Two Waves: Early Phase, Late Phase
Drag the marker across the timeline. Watch which cells and mediators own each window after allergen exposure.
Early
Continuing
Late
0 min15 min1 hr3 hr6 hr
The Effectors and the Crisis
Three cells carry the Type I axis. Tap a card to see what it fires. Then sort a live anaphylaxis in the decision tree.
The lineup
Mast Cell
Tissue-resident, coated in FceRI-bound IgE
HISTAMINE + TRYPTASE + HEPARIN
tap to flip →
Why It Happens
How it fires
Cross-linking of bound IgE aggregates FceRI. Preformed granules release histamine (vasodilation, bronchoconstriction), heparin, and tryptase. It then makes leukotrienes and PGD2 from scratch.
Marker
Serum tryptase is the mast-cell-specific sign of degranulation.
Trap
C3a and C5a can also degranulate it without IgE, so not every mast cell release is allergic.
Basophil
Circulating blood counterpart of the mast cell
HISTAMINE + HEPARIN (no tryptase)
tap to flip →
Why It Happens
How it amplifies
Same FceRI cross-linking logic as the mast cell, but it lives in blood and carries less tryptase. It reinforces systemic reactions.
Role
Amplifies circulating histamine during anaphylaxis and recruits more effectors.
Trap
Mast cell and basophil share IgE biology. The difference is tissue vs blood and the tryptase load.
Eosinophil
Bilobed granulocyte, armed by IL-5
MAJOR BASIC PROTEIN + ADCC
tap to flip →
Why It Happens
How it attacks
Drawn into tissue in the late phase. Releases major basic protein (a helminthotoxin) that shreds parasites but also strips epithelium.
Defense
Kills helminths by antibody-dependent cell-mediated cytotoxicity: IgE or IgG coats the worm, the eosinophil docks and dumps granule enzymes onto it.
Trap
Eosinophilia is a parasite clue, but in allergy the same cell does the late-phase tissue damage.
Visual findings
Urticaria · early-phase wheals · tap to expand
Angioedema · deep swelling · tap to expand
Eosinophil smear · red granules · tap to expand
Skin prick test · wheal and flare · tap to expand
Sort the crisis: acute drug reaction
1
A 52-year-old woman gets IV cefazolin for cellulitis. Within minutes she is short of breath, dizzy, and diffusely itchy. BP 64/38, HR 130, diffuse rash, bilateral wheezing. First decision: what does the time course tell you?
Minutes after exposure points to a preformed-mediator reaction
A slow buildup over hours suggests septic shock
2
Good. Minutes plus itch and wheeze is preformed-mediator release. Now separate the mechanisms. She has urticaria and bronchospasm. Which pathway fits best?
IgE cross-linking on mast cells and basophils
Bradykinin accumulation from an ACE inhibitor
C1 esterase deficiency with low C4
3
Urticaria plus bronchospasm plus minutes is IgE-mediated anaphylaxis. Confirm with the marker and lock the treatment.
Tryptase rises; give IM epinephrine now
Wait for cultures before any treatment
Board Walkthrough
One vignette at a time, drawn from a bank of 25. Cross out and highlight like the real exam. The diagnostic clues glow only after you commit.
VIGNETTE 1 OF 25
Memory Hooks
Tap to unblur. Test yourself before you peek.
⚠
The IgE switch cytokine
"4-E." IL-4 switches to Ig-E. That is the whole isotype-switch question in one syllable.
tap to reveal
☣
The trigger is geometry, not binding
One IgE bound does nothing. The signal is cross-linking: a multivalent allergen bridges two IgE and aggregates FceRI.
tap to reveal
⏱
Two waves, two cells
Minutes = mast cell histamine. Hours = eosinophil major basic protein. Early phase vs late phase in one split.
tap to reveal
📊
The mast cell marker
Tryptase = mast cell degranulation. In systemic anaphylaxis, serum tryptase climbs. Not alkaline phosphatase, not calcitonin.
tap to reveal
🐚
Eosinophil's two jobs
Kills worms by ADCC (IgE or IgG coats the worm, granule enzymes dump onto it). In allergy, the same enzymes shred epithelium in the late phase.
tap to reveal
☀
IL-5 = Eosinophil
IL-5 grows and activates eosinophils. Mepolizumab and reslizumab neutralize IL-5 for eosinophilic asthma.
tap to reveal
💉
Anaphylaxis first move
IM epinephrine, anterolateral thigh, first and fast. Antihistamines and steroids are adjuncts, never the lead.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed July 2026
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
