Which immunoglobulin class is produced in a naïve B lymphocyte that has not yet seen its cognate antigen?

Naïve B lymphocytes predominantly express Immunoglobulin M (IgM) on their surface. In its undifferentiated state, a naïve B cell will typically produce IgM as its first line of antibody-mediated defense before it encounters its specific antigen. This is part of the body's adaptive immune response.

Upon activation by an antigen, these naïve B cells will undergo clonal expansion and may undergo class switching, which leads to the production of other immunoglobulin classes like IgG, IgA, or IgE depending on the cytokine environment and the nature of the immune response. However, until they encounter their cognate antigen, the primary immunoglobulin class they express and produce is IgM.