Background: The neonatal immune system is biased toward tolerance, which is necessary to
prevent attacks on commensal organisms and benign antigens. However, this tolerogenic bias also
leads to overwhelming infection in many neonates each year.
Methods: We review the differences between various components of the neonatal versus adult immune
system and discuss how these changes affect the immune response to pathogens and vaccines.
Results: B cell activity is impaired in neonates, which is partially compensated by acquisition of
maternal antibodies. Regulatory T cells are abundant, and CD4 T cell differentiation is skewed away
from Th1 and toward Th2 and Th17 responses. Effector functions of CD8 T cells and natural killer
cells are less effective, and antigen presenting cells make fewer inflammatory cytokines.
Conclusion: Nearly every component of the neonatal immune system shows differences from adults.
As research progresses, a clearer understanding of the mechanisms of neonatal immune suppression
will allow for the development of therapeutic interventions to help neonates overcome serious