Allophycocyanin (APC)-conjugated anti-mouse primary antibodies are fluorescent immunoreagents that combine the exceptional brightness of the APC phycobiliprotein with the target specificity of immunoglobulins directed against mouse antigens. Because the laboratory mouse (Mus musculus) is the most widely used mammalian model organism in biomedical research, reagents that enable sensitive and reproducible detection of mouse proteins are indispensable across immunology, hematology, stem cell biology, and cancer research.

The term “APC-conjugated anti-mouse primary antibodies” encompasses two distinct reagent formats. Directly conjugated primary antibodies are immunoglobulins that bind a specific mouse antigen (e.g., CD4, CD8, CD14) and carry a covalently attached APC molecule. APC-conjugated anti-mouse secondary antibodies are anti-mouse immunoglobulin reagents (e.g., goat anti-mouse IgG) that bind to a mouse primary antibody and provide the fluorescent signal for indirect detection. Both formats are widely used, and the choice between them depends on the availability of directly conjugated reagents for the target of interest and the experimental design.

The APC Fluorophore

Allophycocyanin (APC) is a phycobiliprotein isolated from the blue-green alga Spirulina (cyanobacteria) and is a member of the phycobiliprotein family of direct fluorescent dyes. APC is a fluorescent light-harvesting protein unique to cyanobacteria and red algae and is a potent donor fluorophore for creating tandem dyes that can be excited off the 633–640 nm laser. APC is isolated from Spirulina and is chemically cross-linked for stability. Cross-linking is essential because native APC dissociates when in diluted solutions or when exposed to chaotropic salts; cross-linked and stabilized APC (XL-APC) has increased stability in the presence of chaotropic salts or at low buffer concentrations compared with native APC.

A key advantage of APC is that after phycobiliproteins are conjugated to secondary antibodies, there is little fluorescence quenching, which results in conjugates of high specific fluorescence compared with conventional fluorophore-antibody conjugates. APC can be excited by light over a wide range of the visible spectrum, is highly water soluble, has a relatively low isoelectric point, and lacks potentially sticky carbohydrate side chains.