In human milk, there is a complex system of antimicrobial factors… most of the factors are produced throughout lactation… Antibodies are present in human milk throughout lactation…
source: Nutrition During Lactation (1991, Institute of Medicine, p. 134-37)
According to the Iowa Extension Service, every teaspoon of breastmilk has 3,000,000 germ killing cells in it; so if a baby gets even one tsp. a day, it is very valuable!
.. it has been shown in man and in several animal models that immunisation via the gut, and also the lungs, stimulates a special population of antibody-producing B lymphocytes. They appear in large numbers in special aggregations in the gut – the Peyer’s patches. These lymphocytes leave the gut after having met bacteria and viruses there and move or “home” to exocrine glands such as the mammary, lacrimal and salivary glands, as well as glands in the mucosal membranes of the bronchi and the gut. As a consequence, human milk contains sIgA antibodies against all those bacteria and viruses which have been in the mother’s gut. This gives the milk capacity to protect against those microorganisms to which the infant is exposed, because they are usually the same as those its mother has been in contact with.
source: L A Hanson et al. Breastfeeding protects against infections and allergy. Breastfeeding Review; Nov l988 , pp l9 – 22.
Some of the immune factors in breastmilk have been shown to increase in concentration as the baby gets older and nurses less, so older babies still receive lots of immune factors. So as a baby starts to nurse less (weaning) and milk supply decreases, the concentration of immunities increases. This isn’t age-dependent, but depends on the amount of milk that baby is removing from the breast. [source: Goldman AS et al. “Immunologic components in human milk during weaning.” Acta Paediatr Scand. 1983 Jan;72(1):133-4.]
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|
Concentration of Immunologic Components in Human Milk
Average Concentration, mg/ml (I’m leaving out the uncertainty factors to make this more readable) |
||||||
|
2-3 days
|
1 mo
|
6 mo
|
12 mo
|
13-15 mo
|
16-24 mo
|
|
| Lactoferrin |
5.3
|
1.9
|
1.4
|
1.0
|
1.1
|
1.2
|
| Secretory IgA |
2
|
1
|
0.5
|
0.8
|
1.1
|
1.1
|
| Lysozyme |
0.09
|
0.02
|
0.25
|
0.196
|
0.244
|
0.187
|
| Sources:
Table 6-5 “Concentration of Immunologic Factors in Human Milk During Several Phases of Lactation” from: Nutrition During Lactation, Institute of Medicine, 1991, p. 134. Table 5-2 “Concentration of immunologic components in human milk collected during second year of lactation” from: Lawrence R and Lawrence R. Breastfeeding: A Guide for the Medical Profession, 5th ed. St. Louis: Mosby, 1999, p. 169 . |
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| Immune factors found in human milk | ||
|
||
| Source: Human milk – Tables of the antimicrobial factors and microbiological contaminants relevant to human milk banking (update) by Dr. John T. May, PhD |
Additional Information
Do mom’s vaccines protect her breastfed baby? @ 
Can Breastfeeding Prevent Illnesses? LLLI FAQ
Breastfeeding Benefits Both Baby and Mom US Centers for Disease Control
How Breast Milk Protects Newborns by Jack Newman, MD, FRCPC
The Mysteries of Milk by Natalie Shenker MRSB
Witkowska-Zimny M, Kaminska-El-Hassan E. Cells of human breast milk Cellular & Molecular Biology Letters 2017; 22:11. doi 10.1186/s11658-017-0042-4
Palmeira P, Carneiro-Sampaio M. Immunology of breast milk. Revista da Associação Médica Brasileira. 2016;62(6):584-593. doi:10.1590/1806-9282.62.06.584.
Goldman AS. The immune system in human milk and the developing infant. Breastfeed Med. 2007 Dec;2(4):195-204. doi: 10.1089/bfm.2007.0024
Field CJ. The immunological components of human milk and their effect on immune development in infants. J Nutr. 2005 Jan;135(1):1-4.
Immune Benefits of Breast Milk at a Glance by Robert J. Huskey
The Newborn Immune System and Immunological Benefits of Breastmilk (excerpts from textbooks)
Goldman AS, Chheda S, and Garofalo R. Evolution of Immunologic Functions of the Mammary Gland and the Postnatal Development of Immunity. Pediatric Research (February 1998):43(2).
Human milk – Tables of the antimicrobial factors and microbiological contaminants relevant to human milk banking (update) by Dr. John T. May, PhD
Anti-inflammatory characteristics of human milk: how, where, why by E. Stephen Buescher, MD, from Adv Exp Med Biol. 2001;501:207-22.
Dai D, Walker WA. Protective nutrients and bacterial colonization in the immature human gut. Adv Pediatr 1999;46:353-82.
Goldman AS. Modulation of the gastrointestinal tract of infants by human milk. Interfaces and interactions. An evolutionary perspective. J Nutr 2000 Feb;130(2S Suppl):426S-431S.
Importance of Glycoconjugates in Breastfeeding and Early Nutrition by Tom Gardiner, PhD, from Glycoscience and Nutrition 2000 July;1(23):1-10.
Kirsi-Marjut Järvinen. Human Milk Immunology in Relation to the Development of Cow’s Milk Allergy in the Breast-fed. Academic Dissertation, September 2000. University of Helsinki, Faculty of Medicine, Institute of Clinical Medicine, Skin and Allergy Hospital, Department of Dermatology.
How the Immune System Develops
Biology of the Immune System from The Merck Manual of Diagnosis and Therapy