What is the primary immune response initiated in fetuses?

In fetuses, the primary immune response is characterized by the production of immunoglobulin M (IgM). This response is crucial in providing early immune defense, as IgM is typically the first antibody that responds to infections and is produced in response to antigen exposure. During fetal development, the immune system begins producing IgM typically around the 20th week of gestation.

IgM is particularly significant because it indicates the fetus's ability to recognize and respond to pathogens, which is critical given that the fetal immune system needs to develop before birth. Unlike IgG, which the fetus acquires from the mother and confers passive immunity, IgM is produced by the fetus itself, showcasing the development of independent immune function.

The other types of immunoglobulins mentioned have distinct roles that are more relevant after the neonatal period. IgG is important for long-term immunity and maternal-fetal transfer, but it is not the primary response seen in fetuses. IgA predominantly functions in mucosal immunity and is more relevant in infants as they are exposed to environmental antigens after birth. IgE is primarily associated with allergic responses and parasitic infections, making it less relevant to the primary immune responses occurring during fetal development.