Low birth weight (LBW) status (< 10% for gestational age at birth) is associated with increased risk for diseases such as type II diabetes mellitus, hypertension, chronic obstructive pulmonary disease and coronary artery disease in adults, and represents one example of the "fetal onset of adult disease" hypothesis. Recent data strongly associates LBW status with impaired innate and adaptive immunity leading to increased risk for severe infections during adolescence or early adulthood. Animal studies suggest that the ratio of certain B lymphocyte subpopulations, the B1a and B1b cells, determines whether deficits in immunity occur.