Social commerce has elevated influencer marketing from a supplementary brand-awareness tactic to a primary transaction-driving channel, yet its effectiveness depends substantially on a psychological variable that resists straightforward measurement: perceived authenticity. This paper examines how influencer authenticity shapes consumer engagement and brand loyalty within social commerce environments, synthesizing empirical research grounded in source credibility theory and parasocial interaction theory. The review traces the mechanism by which perceived authenticity, encompassing consistency, transparency, and relatability, operates through influencer trustworthiness and message credibility to build consumer trust in branded content, drawing on Lou and Yuan's social media influencer value model, which found that trustworthiness and informative content value jointly predict consumer trust and subsequent purchase intention. The study further examines the documented tension between influencer popularity and authenticity, drawing on De Veirman, Cauberghe, and Hudders' experimental findings that higher follower counts increase perceived likeability and popularity but only inconsistently translate into perceived opinion leadership, and situates this tension within the broader micro-influencer literature suggesting that smaller, niche-focused influencers are frequently perceived as more authentic and relationally credible than macro-influencers or celebrities despite their smaller reach. The review also examines sponsorship disclosure research demonstrating that transparent disclosure of paid partnerships can strengthen, rather than undermine, the relationship between authenticity and trust when consumers perceive the disclosed content as genuinely congruent with the influencer's established persona. A comparative methodology is applied using source-credibility-based survey instruments, parasocial interaction scales, and platform engagement metrics reported across the reviewed studies. Findings indicate that authenticity operates as a mediating psychological mechanism rather than a direct driver of purchase behavior, with trust and perceived credibility consistently emerging as the proximate variables through which authenticity translates into engagement and, over repeated exposure, brand loyalty. The paper concludes by discussing the practical tension brands face between engaging high-reach influencers for awareness and high-authenticity micro-influencers for loyalty-building, and identifies persistent measurement and disclosure-related challenges in this rapidly evolving field...