Despite significant advances in scientific research and the advent of treatment and prevention strategies over the past several decades, the HIV/AIDS pandemic continues to pose a significant threat to public health. Nearly 40 million people are estimated to be living with HIV, with 1.5 million new infections in 2021 alone. An effective HIV vaccine would help to reduce the global burden of HIV/AIDS. The humoral or antibody response is one aspect of the immune system that contributes to vaccine-mediated protection from pathogen infection. Studies in animal models have provided proof of concept that HIV antibodies can protect from infection and pathogenesis. However, these models are limited by the use of single challenge viruses and thus do not reflect the diversity of circulating HIV strains. Furthermore, species-specific differences between humans and animal models hinder an accurate assessment of antibody interactions with other arms of the immune system. Studies to identify immune correlates of protection from HIV infection in humans are important for informing the design of an effective HIV vaccine. Vertical transmission of HIV is a natural setting where the role of pre-existing antibodies on HIV acquisition can be directly studied. During gestation, infants receive maternal HIV-specific antibodies via placental antibody transfer and these antibodies remain in circulation after birth. Because HIV is found in breastmilk, breastfeeding infants are exposed to HIV in the presence of pre-existing HIV-specific antibodies. These passive antibodies can be measured directly in infant blood samples. We hypothesize that passively-transferred antibodies contribute to reduced vertical transmission risk. To protect from infection by mediating a myriad of functions, antibodies must bind to specific targets. For HIV, antibodies targeting the Envelope (Env) surface glycoprotein are thought to be critical for protection because Env is required for efficient viral entry. Studies in animal models and humans have consistently demonstrated that antibodies capable of mediating antibody-dependent cellular cytotoxicity (ADCC), or killing of HIV-infected cells, can reduce HIV pathogenesis. Within the setting of breastfeeding transmission, our group previously demonstrated that passively-acquired ADCC correlates with improved survival of infants who acquire HIV, though this study was limited to a single cohort (the NBT cohort). This finding motivates additional studies to better define whether pre-existing ADCC antibodies can protect from infection in addition to reducing pathogenesis. The specific epitopes targeted by protective antibodies are also poorly defined. In this thesis, we sought to bridge these gaps by comprehensively examining whether antibody epitope or ADCC activity protect from HIV acquisition. In Chapter II, we sought to reproduce prior findings of ADCC protection in NBT in a second cohort, CTL. We also used an ELISA that measures antibody potential to engage dimerized FcγRs IIa and IIIa to interrogate the activity of the RFADCC assay used previously. We found that ADCC activity measured via the RFADCC assay correlated with improved survival among infants living with HIV in the CTL and combined NBT/CTL cohorts. Dimeric FcγRIIa or FcγRIIIa binding also correlated with infant survival, indicating that both receptors may mediate the observed survival benefit of pre-existing HIV-specific antibodies. Finally, there was a trend between RFADCC activity, but not dimeric FcγRIIa or FcγRIIIa binding, and reduced infection risk, suggesting that the RFADCC assay may be measuring a distinct activity than the dimeric FcγR ELISA. In Chapter III, we leveraged a high-throughput approach to determine whether specific epitopes targeted by plasma antibodies in breastfeeding mother-infant pairs correlate with reduced vertical transmission risk. We used phage display of HIV peptides from Env that focused on linear epitopes that are less well studied. Although we did not identify any direct correlates of reduced vertical transmission risk, antibodies targeting variable loops 1/2 and constant region 5 (C5) were associated with improved survival of infants who acquired HIV in the NBT cohort. We further validated the C5 results using a peptide ELISA for infants from both the NBT and CTL cohorts, where C5 peptide ELISA activity again correlated with improved survival. C5 peptide ELISA activity also correlated with lower setpoint viral load and delayed HIV acquisition, both predictors of infant survival. Altogether, these results provide new insights into the specificity of pre-existing antibodies that protect from HIV pathogenesis. In Chapter IV, we investigated the specific properties of antibodies comprising a potent plasma ADCC response. We focused on a mother who had high plasma ADCC activity and who did not transmit HIV to her infant despite several high-risk factors. We reconstructed 17 unique antibodies that all mediated ADCC and recognized epitopes across HIV Env. In competition experiments, several antibodies were required to recapitulate the plasma ADCC activity of the mother and her infant, providing evidence for a potent plasma ADCC response that is highly polyclonal. Together, the studies presented in this thesis promote a deeper understanding of the role of antibodies in vertical transmission of HIV. By comprehensively examining both antibody epitope and effector function, this thesis provides a useful framework for studying the impact of multiple properties of antibodies on HIV transmission and pathogenesis. It is clear from the combined studies that antibody epitope and function both contribute to pre-existing antibody-mediated protection from viral pathogenesis.