I’m not sure if you’re like me and like to know why something works the way it does, but if you are and would like to dive into the depths of the science behind OIT, here you are. I personally needed to know to feed my curiosity, feel reassured and most of all to fill my bucket of hope. So let’s dive in!

What causes a food allergy?

In a typical immune system, food is recognized for what it is—food. The body processes it, absorbs the nutrients, and moves on.

In a food allergy, something has gone wrong with that recognition process. The immune system has decided that a particular food—peanut, egg, milk, cashew—is dangerous. Not just unpleasant. Dangerous. A threat. And it has prepared accordingly.

The result is that every time that food shows up, the body doesn’t just process it. It launches a defense—as if an alarm has been set to go off every time that food appears.

Enter IgE—the sensor

IgE is a type of antibody. Antibodies are proteins your immune system makes to recognize and respond to specific targets. They are produced by B cells—white blood cells responsible for making antibodies against specific substances.

In food allergies, some of those B cells have been directed to produce IgE antibodies specific to a food protein—peanut, egg, milk.

Think of IgE as the alarm sensors. Each one is programmed to recognize a specific allergen.

That IgE doesn’t just float around—it attaches to mast cells and basophils, essentially placing those sensors onto cells that are ready to respond.

Mast cells: where the reaction actually happens

Mast cells are found throughout the body, particularly in the gut lining, skin, and airways. They are packed with histamine and other inflammatory chemicals.

If IgE is the sensor, mast cells are the alarm system itself.

When an allergen enters the body, it binds to IgE sitting on mast cells. Because each allergen has multiple binding sites, it can latch onto multiple neighboring IgE molecules at the same time—like a bridge connecting them.

Once that linking happens, the mast cell releases everything it’s been storing. This is called degranulation, and it’s what causes symptoms: histamine causes hives, itching, swelling; other mediators cause airway tightening and digestive symptoms; when multiple systems are involved, that’s anaphylaxis.

This is why reactions are fast—and why they can be so frightening. The alarm isn’t just sensitive—it’s over-sensitive.

So what does OIT actually do?

OIT doesn’t shut the immune system down. It doesn’t rip out the alarm system.

It re-trains it.

When a child begins OIT, IgE levels don’t drop immediately. In fact, they often rise at first—which is a normal sign that the immune system is noticing the allergen. But even though the sensors (IgE) are still there, the alarm doesn’t go off as easily.

The activation threshold often isn’t reached:

• Not enough bridging
• The signal isn’t strong enough
• Mast cells are harder to trigger

At the same time, the gut environment plays a huge role. The digestive system is designed to treat food as safe. So when the allergen shows up repeatedly in tiny, controlled amounts—and nothing bad happens—the body starts learning from that pattern.

It’s as if the system begins to say:
“We’ve seen this before… and nothing happened.”

Regulatory T cells—the immune system’s peacekeepers—begin sending calming signals, telling the system there’s no need to react. 

Over time, IgE becomes less sensitive, less “trigger-happy.” It’s still there—but it’s no longer setting off the alarm at the slightest signal. This is part of why reactions tend to become milder over the course of OIT, even before a child reaches their full maintenance dose.

IgG4 increases—the blocker

At the same time, another major shift is happening.

Over time, the body starts making more IgG4. Instead of driving reactions, IgG4 helps block them before they even start.

These IgG4 antibodies bind to the allergen first—before it ever reaches IgE. In other words, they intercept the signal before it can even reach the alarm.

That means:

• Less bridging
• Less mast cell activation
• Less histamine release

This is one of the most consistent changes we see in OIT.

IgE, on the other hand, behaves differently. It often rises early, and while it can decrease over time, that decrease is slow, inconsistent, and doesn’t happen in every child. What matters more is that the IgE that’s there becomes less reactive.

Over time, as more allergen is intercepted by IgG4, less of it reaches IgE on mast cells. In some children, IgE begin to fall—often much later in treatment, sometimes years into maintenance.

So the pattern tends to look like this:

• IgE rises early
• IgG4 increases
• The allergen is intercepted earlier and earlier
• IgE may slowly decrease over time in some children

So now you have multiple layers of protection:

• The alarm is less sensitive (IgE)
• The responders are calmer (mast cells)
• And the signal is being blocked early (IgG4)

This is what we call desensitization.

Mast cells become less reactive

The third major shift happens at the mast cell level itself.

With repeated, controlled exposure, mast cells adapt. They become less sensitive and less reactive. Even when triggered, they release less histamine.

It’s as if the alarm system itself has been recalibrated—not removed, just turned down.

This is why many children experience fewer symptoms over time, even as doses increase.

What this looks like in practice

All of these changes build gradually and work together.

The allergen enters the body.
IgG4 intercepts much of it.
The IgE that does encounter it is less reactive.
And even if mast cells are triggered, they respond more calmly.

The end result is a higher threshold for reacting.

A child who once reacted to trace exposure may eventually tolerate much larger amounts.

Why reactions can still happen During OIT

OIT isn’t a straight line.

The alarm system is still there—it’s just been retrained.

Certain conditions can make it more sensitive again:

• Illness
• Exercise after dosing
• Missed doses
• Dose increases

This is why dosing rules matter—they’re not arbitrary. They’re designed around how the system behaves.

Desensitization vs tolerance

Desensitization means the immune system responds less reactively to an allergen as long as regular exposure is maintained. In practical terms, this means a child can tolerate a specific amount of the food while continuing daily dosing. If dosing is stopped for a period of time, that protection may decrease.

Tolerance—often called sustained unresponsiveness—means the immune system remains less reactive even after the allergen is no longer being consumed regularly. This outcome is less predictable and does not occur in all individuals.

Most OIT programs are designed to achieve desensitization. For many families, this provides meaningful protection against accidental exposures and can significantly reduce the risk of severe reactions.

The bottom line

When I understood the biology, OIT made so much more sense. The daily doses aren’t just a protocol to follow—each one is a lesson for the immune system. Each updose is a slightly harder lesson. And the consistency, the food timing, the calm after dosing—it all maps directly to the biology.

It also helped me feel less scared on the hard days. When my daughter had a stomach ache after a dose, I understood why. When we had to downdose after she was sick, I understood that too. The science made us better at navigating this journey.

I hope it does the same for you.

Sources & Further Reading

Wood RA et al.
Oral Immunotherapy for Food Allergy
https://www.jiaci.org/revistas/vol27issue3_1.pdf

Vickery BP et al. (2014)
Sustained unresponsiveness to peanut in subjects who have completed peanut oral immunotherapy
https://pubmed.ncbi.nlm.nih.gov/24361082/

Vickery BP et al. (2017)
Early oral immunotherapy in peanut-allergic preschool children is safe and highly effective
https://pubmed.ncbi.nlm.nih.gov/27522159/

Nowak-Wegrzyn A et al.
Mechanisms of Oral Immunotherapy
https://pmc.ncbi.nlm.nih.gov/articles/PMC9362513/

Nurmatov U et al.
Allergen Immunotherapy for Food Allergy: Systematic Review
https://doi.org/10.1111/all.13124

Jutel M & Akdis CA (2011)
Immunological mechanisms of allergen-specific immunotherapy
https://pubmed.ncbi.nlm.nih.gov/21466562/

Best wishes,

Sarah

OIT Mama