the polysaccharide to a protein carrier, a T cell response can be induced. Normally, polysaccharides by themselves cannot be loaded onto the major histocompatibility complex (MHC) of antigen presenting cells (APC) because MHC can only bind peptides. In the case of a conjugate vaccine, the carrier peptide linked to the polysaccharide target antigen is able to be presented on the MHC molecule and the T cell can be activated. This improves the vaccine as T cells stimulate a more vigorous immune response and also promote a more rapid and long-lasting immunologic memory. The conjugation of polysaccharide target antigen to the carrier protein also increases efficiency of the vaccine, as a non conjugated vaccine against the polysaccharide antigen is not effective in young children. The immune systems of young children are not able to recognize the antigen as the polysaccharide covering disguises the antigen. By combining the bacterial polysaccharide with another antigen, the immune system is able to respond. Approved conjugate vaccines The most commonly used conjugate vaccine is the Hib conjugate vaccine. Other pathogens that are combined in a conjugate vaccine to increase an immune response are Streptococcus pneumoniae (see pneumococcal conjugate vaccine) and Neisseria meningitidis (see meningococcal vaccine), both of which are conjugated to protein carriers like those used in the Hib conjugate vaccine. Both Streptococcus pneumoniae and Neisseria meningitidis are similar to Hib in that infection can lead to meningitis. In 2018, World Health Organization recommended the use of the typhoid conjugate vaccine which may be more effective and prevents typhoid fever in many children under the age of five years. In 2021, Soberana 02, a conjugate COVID-19 vaccine developed in Cuba, was given emergency use authorisation in Cuba and Iran. Select list of other conjugate vaccines Various immunocontraception vaccines for animal use, including GonaCon (GnRH linked to keyhole limpet hemocyanin) NicVAX, which aims to vaccinate against nicotine using a chemically modified hapten version linked to exotoxin A TA-CD, cocaine linked to inactivated cholera toxin TA-NIC, nicotine linked to inactivated cholera toxin See also Toxoid Vaccine T cell B cell Immunogenicity Immune system Immune response References External links Vaccines Vaccination