The first antibody to appear in the serum following stimulation by antigen is

IgM antibodies are the first to form on a growing B cell's surface.
It's also the most common type of antibody produced into the bloodstream during the early phases of a primary antibody response, such as when a person is first exposed to an antigen.
IgM antibody is the first to develop, followed by IgA on mucosal surfaces and IgG in the blood.
As a result, IgM is the first antibody to emerge in serum after antigen stimulation.