Compared to all other isotypes, soluble IgE is in very low concentrations in the blood. This is because:

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The need for IgE is very rare and so it is only produced in low concentrations

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IgE is mainly secreted across the gut lumen and so is not found in the blood

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Similar to the alternative complement pathway, IgE spontaneously binds to the surface of molecules and is removed from circulation

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The affinity of the IgE Fc receptor is the highest of all the Fc receptors

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