As a scientist working in the fields of lactation, immunology and
nutrition, I would caution everyone not to make too much of the recent
study finding a difference in thymus size between breast-fed and non
breast-fed infants.  While the possiblities are interesting, we do not
know what effect this difference would have on "real-life" outcomes like
cancer risk, infectious disease resistance and autoimmune disease
susceptability.  To make an analogy, if we found that non breast-fed
infants weighed more than breast-fed infants of the same age, we would
not know if the difference was due to fatness, increased muscle mass or
extra retained water - any one of which would change our perception of
the study.

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J. Paul Zimmer, Ph.D.
Developmental and Clinical Immunology
University of Alabama at Birmingham