Growing evidence indicates that the gut microbiota is a central regulator of systemic immunity, acting through epithelial barrier integrity, microbial metabolites, and bidirectional signaling with innate and adaptive immune cells. Within this framework, probiotics have attracted substantial interest as tools for immune modulation; however, their effects are not uniform and should not be generalized across species or formulations. This review synthesizes current evidence on the gut microbiota–immune axis and examines how defined probiotic strains influence immune homeostasis, inflammation, and clinical outcomes. Particular emphasis is placed on strain-specific effects among lactic acid bacteria, bifidobacteria, yeast probiotics, and emerging nontraditional candidates, with attention to mechanisms involving cytokine signaling, regulatory T-cell induction, nuclear factor kappa B (NF-κB) modulation, toll-like receptor (TLR) pathways, short-chain fatty acids (SCFAs), tryptophan metabolites, and bile-acid-dependent signaling. The available literature indicates that the most meaningful immunological effects arise from precisely characterized strains acting in specific host contexts, whereas inconsistent trial design, small sample sizes, variable dosing, and poor strain resolution continue to limit translation. Overall, current data support a shift from generic probiotic use toward mechanism-based, strain-specific, and increasingly personalized strategies for immune modulation.