Curved Scatchard plot above linear line indicates
An upward-curving Scatchard plot that lies above the linear reference line is the classic signature of positive cooperativity. Binding of the first ligand molecule increases the affinity of remaining sites, producing a concave-up shape. Negative cooperativity or multiple independent sites of differing affinity generate downward curvature. Additional methods such as Hill analysis are often required to distinguish these possibilities.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)