Many moms wonder whether their babies will be protected from any illness that mom has been immunized against as long as breastfeeding continues. Breastfeeding will enhance baby’s response to immunizations that he receives, however, breastfeeding will not act as a substitute for immunization.
The immunities that our bodies generate when we get an illness or receive a vaccination are IgG immunities. IgG is the major immunoglobulin circulating in the blood and is the type of antibody that provides long-term resistance to illnesses – the IgG antibodies ‘recognize’ germs that we’ve been exposed to previously so that they can be destroyed more quickly.
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IgG protection from mom primarily comes to her baby via the placenta prior to birth –this maternal IgG in baby’s system gradually disappears by 6-8 months postpartum. A child’s own IgG synthesis gradually increases until it reaches adult levels by 7-8 years of age [ref: Behrman: Nelson Textbook of Pediatrics, 16th ed., Copyright © 2000 W. B. Saunders Company. p. 595]. The child continues to get some IgG protection from breastmilk for as long as nursing continues, but IgG does not enter the breastmilk in quantities high enough to “vaccinate” baby (though it’s certainly helpful).
Immunities that babies receive from nursing are primarily IgA (IgG and other immunoglobulins are present in much lower quantities). IgA concentrates in body fluids such as tears, saliva, and the secretions of the respiratory and gastrointestinal tracts and has an important protective function.
The many immune components in breastmilk can make a huge difference when it comes to keeping baby healthy. Even though baby does not receive enough of mom’s IgG immunities via breastmilk to qualify as an immunization against a particular illness, there are many other immunities (IgA, certain fatty acids, etc) in the breastmilk that are active against the same illnesses.
As an example, what protection does baby get from the chicken pox (varicella) virus if mom had chicken pox as a child?First off, baby would not be expected to be immune to chicken pox, particularly after 6-8 months when placental immunity has faded away. However, other immune factors in breastmilk will give baby some protection from chicken pox. Per Dr. Jack Newman (in How Breast Milk Protects Newborns), “Free fatty acids present in milk can damage the membranes of enveloped viruses, such as the chicken pox virus, which are packets of genetic material encased in protein shells.” The secretory IgA in breastmilk has also been shown to be active against the chicken pox virus in vitro. Case reports suggest that–as with other viral infections–breastfed babies who get chicken pox will often (but not always) get milder cases.
Additional Information
Immune factors in human milk @ 
The Immune System and Primary Immune Deficiency Diseases from The Immune Deficiency Foundation Patient and Family Handbook For The Primary Immune Deficiency Diseases, Fourth Edition
Understanding The Immune System from the National Institute of Health
Hahn-Zoric M, Fulconis F, Minoli I, et al. Antibody responses to parenteral and oral vaccines are impaired by conventional and low-protein formulas as compared to breast feeding. Acta Paediatr Scand 1990;79:1137–42.
“Host-resistance factors and immunologic significance of human milk” from: Lawrence R and Lawrence R. Breastfeeding: A Guide for the Medical Profession, 5th ed. St. Louis: Mosby, 1999, p. 159-196.
The Immune System. In: Riordan J. Breastfeeding and Human Lactation, 3rd ed. Boston and London: Jones and Bartlett, 2004, p. 117-121.
Smith, LJ. Allergenic Protection and Defense Agents and Systems in Human Milk. In: Walker M, ed. Core Curriculum for Lactation Consultant Practice. Boston: Jones and Bartlett, 2002, p. 118-155.