This is a simplified explanation of CARPA – there are more resources available at the bottom of the page for further reading for those who are interested in knowing more.
What does this term mean?
System is the name of a group of proteins which circulate in the blood and tissue fluids. They are activated by the presence of molecular components they don’t recognise. This can be bacteria or viruses, but also damaged human cells and drug components they have never encountered before.
The proteins become sequentially activated in an enzyme cascade which can trigger an inflammatory immune response. This is what the term Activation-Related
means that the reaction is not a genuine IgE allergy but can present itself as one. A genuine allergy means that the body reacts in the same or similar way every time it is exposed to a specific allergen and can be detected by an IgE blood test. A pseudo-allergic reaction can be a single incident that would not be repeated on a subsequent exposure to the same allergen and would not show any sensitisation in an IgE blood test.
Why does the body react this way?
The body mounts an immune response against what it considers to be harmful biological and chemical agents.
There are 3 ways a reaction might take place and each is distinct in how the symptoms appear, how long after ingestion/injection they appear as well as dose dependence and cross-reactivity.
The drug binds to proteins already present and forms new antigens causing an allergic immune reaction.
The drug has off-target activity (e.g., if a drug was meant to target a bacterial infection, but instead caused an inflammatory response in the skin).
The drug stimulates or inhibits receptors or enzymes of inflammatory cells.
What things cause these reactions?
Vaccines, various medications, occasionally food additives, preservatives, colouring and artificial flavours have all been noted to cause these types of pseudo-allergic reactions.
What symptoms do they cause?
Most commonly CARPA reactions involve the skin. Generalised reactions could cause urticaria (hives or welts), angioedema (swelling under the skin), non-allergic asthma and eczema flare ups.
Localised reactions could cause abscesses (lumps containing pus), nodules (lumps under the skin) or granulomas (small lumps), this is more likely the case with intravenous drugs, vaccines and insect bites.
Occasionally a pseudo-allergic reaction can be anaphylactic in nature which is what would cause a misdiagnosis of a true allergy.
What is the importance of knowing whether a reaction is CARPA or a true allergy?
Misidentifying a singular incident as an allergy can sometimes lead to treating subsequent illnesses with medicines that are not optimised for recovery due to a perceived allergy. The most cited example of this is penicillin allergy, it is thought that in the US that 10% of patients have a penicillin allergy recorded, but the true number of those with an actual allergy is much lower. A noted penicillin allergy can lead to alternative, but less effective, antibiotics being used and would rule out a whole group of medicines which are penicillin related for treatment.
Multiple allergies are becoming more common and this often leads people to impose a strict restrictive diet on themselves. This can lead to a poor diet lacking in essential nutrients and frustration over a lack of eating options. Knowing which reactions are genuinely allergic reactions and which are a single incident of CARPA can bring more options back into a diet.
This is why food diaries continue to be an important tool in diagnosis of your allergies – noting the times reactions took place and what medications were taken are a necessary starting point for a proper diagnosis.
for a free to download food diary with tips on how to complete or follow the link to the paperback Allergy Resources Food Diary
Let me know if you found any of these interesting or useful.
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