Any of a class of proteins present in the serum and cells of the immune system, which function as antibodies.
‘she was given intravenous immunoglobulin’
count noun ‘they are susceptible to infection in spite of high concentrations of immunoglobulins’

Immunoglobulin | Definition of Immunoglobulin by Lexico
I find it difficult to differentiate the use of the noun "immunoglobulin" as mass and countable noun.
Jigneshbharati"immunoglobulin" as mass and countable noun.

As a mass noun it refers to the liquid mixture that is used intravenously.
As a count noun it refers to the individual molecules of protein or to individual kinds of immunoglobulin proteins.

Roughly speaking, mass nouns are analogous to liquids; count nouns are analogous to objects.