Using the quantitative proteomics workflows and statistics for milk analysis, we identified that protein kinase C-binding protein NELL2, thrombospondin-1, and complement factor I have diagnostic potential for differentiating staphylococci and streptococci intramammary natural infection and inflammation, and subsequently gained a deeper understanding of the immunopathology of mastitis-related infection and inflammation for various etiological agents. The gene discussed is NELL2; the disease is infection.