Which immunoglobulin appears first in the primary immune response?

In the context of the primary immune response, immunoglobulin M (IgM) is the first antibody that is produced by the immune system upon initial exposure to an antigen. IgM plays a crucial role during the early stages of this response due to its pentameric form, which allows it to effectively bind multiple antigens and form complexes. This characteristic is significant for the agglutination of pathogens and providing a strong first line of defense.

As the primary immune response progresses and the body begins to produce more specific antibodies, immunoglobulin G (IgG) will typically follow IgM in significant amounts, particularly in secondary responses and after class switching has occurred. However, during the initial phase, IgM is crucial for its early production and function.

The other immunoglobulins listed do not play the same role at this initial phase. IgA is primarily involved in mucosal immunity, IgE is associated with allergic reactions and parasitic infections, and both appear later or in specific contexts within immune responses. Thus, the presence of IgM marks the beginning of the immune response to a new infection.