With four subclasses, which immunoglobulin is the predominant antibody in the secondary immune response and has the greatest concentration of the five immunoglobulin classes in the fetus?

IgG antibody provides an "immune history." That is, IgG antibody persists in most people and indicates the antigens to which they have been exposed. IgG is not formed early in infection but is a secondary response arising weeks to months after antigenic challenge. IgG also has a built-in memory. Even people with very low levels of specific IgG will respond to an antigen challenge with an IgG response.