Immunology is a fascinating and sometimes confusing topic. This is especially true when considering how the immune system can seemingly turn on its host, causing an allergic response.
Typically, the body will create antibodies against harmful foreign material, such as a virus. Some individuals will create antibodies against foreign material that is not harmful, such as a food antigen.
The Most Severe Allergic Reaction: Anaphylactic & Anaphylactoid
When you typically think of an allergic reaction, you think of anaphylactic shock. The antigen reacts with IgE antibodies which trigger mast cell degranulation (cell granules secreting compounds) and histamine release. This leads to airway constriction, low blood pressure, a fast heartbeat, and respiratory distress.
Common triggers include:
Symptoms commonly occur immediately or within 20 minutes of exposure. However, delayed response up to several hours may occur after oral ingestion.  Very rare cases report delayed reactions up to 24 hours. This is also the case for most IgE-mediated responses (hives, swelling, vomiting, etc.), read more about types of infant food allergies here.
The non-immune mediated version of anaphylactic reactions is known as an anaphylactoid reaction. The antigen instead causes a direct histamine release not related to IgE antibodies or mast cell degranulation. 
Since IgE antibodies are not involved, an anaphylactoid reaction can occur upon first exposure, whereas anaphylaxis requires prior sensitization or cross-reactivity (think: reacting to latex the first time after being exposed to banana which is very similar).  This type of reaction also cannot be tested via typical IgE-based skin prick and blood tests. Symptoms commonly occur immediately or delayed a few hours after oral ingestion as in anaphylactic responses.
The Other Allergic Response: Non-IgE Allergic Reactions
Many reactions can occur through the same non-IgE mediated pathway described above.
Possible non-IgE food allergy symptoms: 
Mucous in stool
Failure to Thrive
Lethargy & More
These are different from intolerances, in that intolerance is due to low enzyme levels causing the inability to properly digest a particular food molecular, ssuch as lactose, while a non-IgE mediated allergy is an immune response.  Non-IgE allergies are difficult to diagnose due to delayed symptoms and a lack of easily accessible diagnostic measures. 
This type of reaction commonly affects the gastrointestinal (gut) mucosa. This mucosal barrier is designed to balance digestion, absorption, cohabiting beneficial flora, and maintaining pathogenic immune defense all at once. This balance is developed during infancy which aligns with early patient onset and then often “outgrowing” the allergy later in life.  This immune and gastrointestinal system development explains why non-IgE is much more likely to resolve over time compared to IgE allergies.
When Will You See a Reaction?
Non-IgE reaction timing varies widely. Some occur within an hour of exposure and others take up to 48 hours, depending on the child’s sensitivity levels and the type of reaction. If symptoms are severe in your baby it can take up to 10 days.
We know this is hard to swallow as a parent, but our food allergy experts (and parents) are here to guide you on this confusing journey. Our consults are tailored to meet your family’s needs and get you fast results!
Chronic Non-IgE Reactions + Breastfeeding?
Many parents are told that it takes upwards of 2 weeks or more for ingested proteins to clear from breast milk.
Research outlined in our article here indicates that timeline is less than 24 hours. What gives?
Let us look first at children who are immediately switched to a hypoallergenic formula post-reaction. Those children continue to experience symptoms for 3-10 days or longer in some cases as their bodies heal. While an acute reaction will see immediate improvements, a chronic one will need much more healing time.