There are four IgG subclasses:IgG1, IgG2, IgG3, and IgG4. Each subclass has distinct structural characteristics and is present in different concentrations in the blood. Each IgG subclass also serves a different (but not exclusively different) function, which contributes to the diverse immune responses against pathogens and antigens.

In the bloodstream, the total IgG content is comprised of 60-70% IgG1, 20-30% IgG2, 5-8% IgG3, and 1-3% IgG4. These percentages vary by age, with IgG1 and IgG3 subclasses reaching normal adult levels by 5-7 years of age. Levels of IgG2 and IgG4 subclasses rise more slowly and reach adult levels only at about 10 years of age.

With regards to function: 

• IgG1 and IgG3: Protect against viral infections and toxins from bacteria such as tetanus and diphtheria
• IgG2: Primarily defends against the polysaccharide capsules found on certain disease-causing bacteria, such as Haemophilus influenza and Streptococcus pneumoniae
• IgG4: Unique structural and functional characteristics suggest anti-inflammatory and tolerance-inducing effects