Which immunoglobulin is found scarce in human serum?
There are five classes of immunoglobulin (Ig): IgG, IgM, IgA, IgD and IgE.
In normal serum, about 80% is IgG, 15% is IgA, 5% is IgM, 0.2% is IgD and a trace is IgE.
IgE is mainly produced in the linings of respiratory and intestinal tracts. Serum contains only traces (a few nanograms per ml). 
It is mostly distributed extravascularly.
It is also referred to as reagins. 
Molecular weight is 1,90,000 (8S molecule). 
Half life is 2-3 days.
It resembles IgG in structure.
It is heat labile (inactivated at 56°C in one hour) whereas other immunoglobulins are heat stable.
It has affinity for surface of tissue cells, particularly mast cells of the same  species (homocytotropism).
IgE mediates type I hypersensitivity (atopic) reaction.This is responsible for asthma, hay fever, eczema and Prausnitz-Kustner (PK) reaction.
High level of IgE in serum is also seen in children with a high load of intestinal parasitism. 
It cannot cross the placental barrier or fix the complement.
IgE is responsible for anaphylactic type of reaction. 
It may play some unidentified role in defence against intestinal parasitic infections

Ref : textbook of microbiology, C P Baveja, 5th edition , Pg no:102
IgG
IgM
IgE
IgA