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Breastmilk is a complex biological fluid containing macromolecules including lipids, oligosaccharides, proteins as well as several types of cells. Several studies have reported in detail of these components. My focus is studying the protein component of breastmilk, specifically antibodies, cytokines, and other secreted factors in the setting of different pathogenicity in women. Studies have demonstrated that the levels of numerous cytokines as well as the levels of pathogen-specific antibodies are altered in milk upon either maternal or infant infections. Additionally, there are reports that the levels of certain inflammatory markers are altered in milk among women with breast cancer or at increased risk of breast cancer. In my thesis, I sought to elucidate the inflammatory profile of breastmilk obtained from women who: (1) were infected and/or recovered from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), (2) were vaccinated against the disease caused by SARS-CoV-2, coronavirus disease 2019 (COVID-19), and (3) were at high risk of breast cancer because of germline BRCA mutations. In my first study, I evaluated the immune response to SARS-CoV-2 in colostrum (the first secretion from the mammary gland after birth) from women who tested positive for the virus at delivery or during pregnancy. My focus was on trying to better understand the antibody and cytokine responses among the 15 enrolled women. The first study presents a proof-of-concept on how colostrum collected on paper spot cards can serve as a reliable surrogate for liquid samples. Because women express very small volumes of colostrum, in the first study, I found that colostrum from more than 70% of the women contained detectable levels of anti-SARS-CoV-2 antibodies. One of my main findings was that comparable levels of antibodies against SARS-CoV-2 were present in colostrum expressed by women testing positive for the virus over four months ago and women testing positive at delivery. I concluded that a strong humoral response is present in the colostrum of women with an active infection and women who recovered from it. I also provide evidence of how future large-scale studies can be conducted with milk easily collected on paper spot cards. However, the durability of antibody responses and the presence of other immune cells needed to be determined. In my next COVID-19 study, I analyzed serial milk obtained from 30 women collected over a period of 35 days, with an additional sample collected at around 4 months after a positive COVID-19 test. Milk from nearly 90% of women contained detectable levels of anti-SARS-CoV-2 antibodies. Importantly, I found that the levels of a specific subtype of T-cells, called effector-memory T cells, are upregulated in milk expressed when women recovered from COVID-19. Additionally, antibodies in milk were able to neutralize the virus and select variants of concern. Both the studies informed that SARS-CoV-2 infection elicits a robust humoral and cellular response, with the humoral response predominantly IgA-driven. In my next study, I determined whether these responses differed among women who received an mRNA-based COVID-19 vaccine. In this longitudinal study of 30 enrolled lactating women, I found that the humoral response is IgG-driven. In fact, there was no significant difference in the levels of anti-receptor binding domain (RBD)-IgA between milk expressed prior to vaccination and milk expressed three weeks after the second dose. To determine whether RBD-specific antibodies were vertically transmitted via breastmilk to infants, I measured anti-RBD IgG and IgA in stool samples from infants of vaccinated mothers. A third of these infant samples contained low, but detectable levels of antibodies, providing compelling evidence that breastmilk-derived antibodies are indeed being transmitted to breastfed infants. In my pursuit to profile immune markers in breastmilk, I assessed the levels of a panel of immune biomarkers in the milk of women with a pathogenic germline BRCA mutation. A total of 202 milk samples from BRCA carriers and 78 milk samples from non-carrier controls were analyzed. I hypothesized that the levels of these biomarkers would be significantly different in the milk of BRCA carriers compared to non-carrier controls. I found that the levels of certain inflammatory cytokines are elevated whereas levels of the member of the cytokine superfamily, osteoprotegerin (OPG) was lower in BRCA carriers compared to controls. Intriguingly, the alteration of these biomarkers was significant among BRCA1 carriers and not BRCA2 carriers, who more resembled the non-carrier controls for the panel assayed. Assessment of these biomarkers in women at high risk of breast cancer may be useful in assessing individual risk and in developing effective prevention measures.