CD4 and bacterial infectious disease: The polysaccharide conjugate vaccines are based on bacterial capsular polysaccharides conjugated with a carrier protein. The polysaccharides alone cannot be processed and displayed on MHC molecules like proteins, consequently, it activates an immune response T-cell independent. The polysaccharide conjugation with a protein carrier enhances the engagement of CD4+ T-helper cells through the interaction with the MHC class II of B-cells which leads to higher affinity and class-switched antibodies.Efficient for vaccination against extracellular bacterial infection.