18 month old had bloodwork done following allergy testing. Her IgE levels were low and they want to repeat test in 4 wks. What does low IgE cause?

Nothing. Low ige is "normal". If the concern is that she has symptoms of allergies and a low ige they want to see if it goes up in 4 weeks which would be more suggestive of allergies. Elevated ige is sign of allergies. A low ige is actually normal. Under 4 years old children's allergies change dramatically every 6 months so sometimes we need to follow with serial tests...
Nothing per se. Low ige does not cause anything per se. It may be a sign of an underlying disorder. You really should get this answer from the doc who is drawing the testing.
Nothing. High ige would have been an indication of an allergic response. Low ige would indicate a lack of or minimal allergic response.