A conjugate vaccine is a type of subunit vaccine which combines a weak antigen with a strong antigen as a carrier so that the immune system has a stronger response to the weak antigen. Vaccines are used to prevent diseases by invoking an immune response to an antigen, part of a bacterium or virus that the immune system recognizes. This is usually accomplished with an attenuated or dead version of a pathogenic bacterium or virus in the vaccine, so that the immune system can recognize the antigen later in life. Most vaccines contain a single antigen that the body will recognize. However, the antigen of some pathogens does not elicit a strong response from the immune system, so a vaccination against this weak antigen would not protect the person later in life. In this case, a conjugate vaccine is used in order to invoke an immune system response against the weak antigen. In a conjugate vaccine, the weak antigen is covalently attached to a strong antigen, thereby eliciting a stronger immunological response to the weak antigen. Most commonly, the weak antigen is a polysaccharide that is attached to strong protein antigen. However, peptide/protein and protein/protein conjugates have also been developed. History The idea of a conjugate vaccine first appeared in experiments involving rabbits in 1927, when the immune response to the Streptococcus pneumoniae type 3 polysaccharide antigen was increased by combining the polysaccharide antigen with a protein carrier. The first conjugate vaccine used in humans became available in 1987. This was the Haemophilus influenzae type b (Hib) conjugate, which protects against meningitis. The vaccine was soon incorporated with the schedule for infant immunization in the United States. The Hib conjugate vaccine is combined with one of several different carrier proteins, such as the diphtheria toxoid or the tetanus toxoid. Soon after the vaccine was made available the rates of Hib infection dropped, with a decrease of 90.7% between 1987 and 1991. Infection rates diminished even more once the vaccine was made available for infants. Technique Vaccines evoke an immune response to an antigen, and the immune system reacts by producing T cells and antibodies. The B memory cells remember the antigen so that if the body encounters it later, antibodies can be produced by B cells to break down the antigen. For bacteria with a polysaccharide coating, the immune response creates B cells independent of T cell stimulation. By conjugating