What is the isotype in the following?
A 26-year old primigravid woman delivers a 2948-g (6-lb 8-oz) male newborn at term. Six months ago, she received a vaccine, Streptococcus agalactiae (group B), serotype III, during a clinical trial. Screening indicated that she had generated an antibody response to this polysaccharide antigen within 1 month of the vaccine. This antibody was detected in her serum immediately prior to the onset of labor. Eight hours after delivery, the newborn appears progressively lethargic, and his temperature is 38.2C (100.8F). Cerebrospinal fluid and blood cultures grow S. agalactiae (group B), serotype III. It is suspected that the mother produced only one antibody isotype in response to the vaccine.