During a primary immune response, B cells initially produce IgM antibodies because IgM is the first immunoglobulin expressed before class switching occurs. IgM has a pentameric structure that allows effective agglutination and complement activation, providing early defense against pathogens. Other isotypes like IgG, IgA, and IgE appear later after class switching guided by cytokines and antigen stimulation.

Reference: Janeway's Immunobiology, 9th Edition, Chapter 5: The Humoral Immune Response, Page 190