Polyclonal Antibody in serum or affinity purified, what is a difference and how can it influence my results?

Catalog and custom-produced Polyclonal Antibodies can be provided in different formats, such as: 

Serum (1)
Contains a mixture of various immunoglobulins against different antigens (met by an animal during immunization course). The concentration of specific antibody in serum can be low, and usually varies between 0.5 to 5 % of the total IgG pool (concentration of IgG varies from 10 to 20 mg/ml between immunized animals). 
Information presented on the  antibody product information sheet: 
50, 100 or 200 µl serum

Affinity purified using Protein A or G (2)
This procedure will yield a total immunoglobulin fraction. Antibodies specific to a given target will vary from 0.5 to 5 % of such total IgG pool. 
Information presented on the  antibody product information sheet: 
100 or 200 µg

Affinity purified using specific antigen (protein, peptide) (3) 
This procedure is done using a matix, coupled with a specific protein or peptide used to elicit the given antibody. This will result in a specific antibody of >80 % purity, depending on the procedure.  
Information presented on the antibody product information sheet: 
50, 100 or 200 µg

A product which is provided as 50 µl of serum (1), or 50 µg of serum purified on Protein A or G (2), will not contain the same amount of specific antibody as 50 µg of antibody purified using the specific antigen (3).