Therefore, to stimulate T-cell responses, early attempts were focused on developing carbohydrate-protein conjugate vaccines by conjugating synthetic TACAs with a carrier protein (Figure 2), such as keyhole limpet hemocyanin (KLH), bovine serum albumin (BSA), diphtheria toxoid (DT), tetanus toxoid (TT), human serum albumin (HSA), ovalbumin, meningococcal outer membrane protein complex (OMPC), Hemophilus influenzae protein D, Pseudomonas aeruginosa exotoxin A (rEPA), etc. [24,33]. Here, ALB is linked to diphtheria.