to T-cell dependent, leading to increased immunogenicity in infants and antibody booster response to multiple doses of vaccine. The first conjugated polysaccharide vaccine was for Haemophilus influenzae type b (Hib). Also now available are conjugate vaccines for pneumococcal disease and meningococcal disease. Recombinant vaccines Vaccine antigens may also be produced by genetic engineering technology. These products are sometimes referred to as recombinant vaccines. There are four genetically-engineered vaccines are currently available: Hepatitis B vaccines are produced by insertion of a segment of the hepatitis B virus gene into the gene of a yeast cell. The modified yeast cell produces pure hepatitis B surface antigen when it grows. Human papillomavirus vaccines are produced by inserting genes for a viral coat protein into either yeast (as the hepatitis B vaccines) or into insect cell lines. Viral-like particles are produced and these indice a protective immune response. Live typhoid vaccine (Ty21a) is Salmonella typhi bacteria that has been genetically modified to not cause illness. Live attenuated influenza vaccine (LAIV) has been engineered to replicate effectively in the mucosa of the nasopharynx but not in the lungs. General Principles of the action of vaccines Most vaccines are injected directly into muscle tissue. Briefly the following occurs*: Cells of the non-specific immune system (i.e. macrophages and dendritic cells) recognise the antigen as foreign and engulf it. These cells then chop the antigen into smaller fragments and display these on their cell surfaces. The dendritic cells move through the lymphatic system to a local lymph node