It's just your body responding to a foreign protein.
Interferon Beta-1A is not a foreign protein. It is molecularly identical to the interferon beta that's naturally produced in the body. You can access the Drs. Prescribing Information from here
AVONEX ® (Interferon beta-1a) is a 166 amino acid glycoprotein with a predicted molecular weight of approximately 22,500 daltons. It is produced by recombinant DNA technology using genetically engineered Chinese Hamster Ovary cells into which the human interferon beta gene has been introduced. The amino acid sequence of AVONEX ® is identical to that of natural human interferon beta.
Natural interferon beta and Interferon beta-1a are glycosylated, with each containing a single N-linked complex carbohydrate moiety.
Both of these characteristics make it different from Betaseron
, interferon beta-1b, which is not identical sequence wise and is also not glycosylated since it's produced in bacteria. This lack of glycosylation in Betaseron may likely be the characteristic which is responsible for the higher probability of forming neutralizing antibodies against Betaseron as opposed to a lower incidence with Avonex.
In summary, Avonex is identical to natural interferon beta and is not a foreign protein.