Group A carbohydrate antigen (GAC)

Group A carbohydrate antigen (GAC) is a highly conserved, surface-exposed polysaccharide uniquely characteristic of Group A *Streptococcus* (GAS; *Streptococcus pyogenes*) and named as the Lancefield group antigen. GAC consists of a polyrhamnose backbone with immunodominant *N*-acetylglucosamine (GlcNAc) side chains (roughly every other rhamnose unit), which may be further modified by glycerol phosphate on a proportion of residues. GAC comprises about 30–60% of the cell wall by mass, providing structural integrity, maintaining cell morphology, and facilitating cell division. It is central to GAS virulence, serving as a key protective barrier and conferring resistance to host immunity by limiting the effectiveness of zinc toxicity and neutrophil-mediated killing. GAC is the primary antigen detected in rapid diagnostic tests for GAS infections and is under active study as a vaccine target due to its surface exposure, conservation, and immunogenicity. Safety challenges for vaccine development arise from concerns about antibody cross-reactivity to GlcNAc, which may contribute to autoimmune sequelae such as rheumatic fever; thus, modified or truncated forms of GAC are being explored for safer vaccine design[3][5][6][7].

Other names

Group A carbohydrateLancefield group A carbohydrateGACStreptococcal group A antigen

Induces antibody response for *Streptococcus pyogenes* diagnosis and immunity[3][5]. Proposed for prevention via vaccine-induced immunity[5][6].