IgG₃ is the dominant subtype of anti-isoniazid antibodies in patients with isoniazid-induced liver failure

Isoniazid (INH) therapy is associated with a significant incidence of idiosyncratic liver failure. We recently reported eight cases of INH-induced liver failure in which patients had antidrug and anticytochrome P450 antibodies. However, it was unclear what role these antibodies play in the mechanism of INH-induced liver injury. Here, we report that the dominant isotype of anti-INH antibodies was IgG, with IgG3 being the dominant subtype. IgG3 antibodies are associated with a Th1-type immune response and fix complement. IgG3 antibodies have been associated with other forms of liver injury and may play a pathogenic role in INH-induced liver injury.