Vaccines have been used for centuries to immunize individuals against pathogenic organisms with the goal of preventing the associated diseases. Vaccines are biological products that contain antigens capable of inducing a specific and active acquired immune response in the body. Antigens present in vaccines are processed by specialized cells in the body's immune system, resulting in the development of blood proteins known as antibodies (i.e., humoral immunity), specialized lymphocytes (i.e., cell-mediated immunity), or both. Therefore, immune responses may be antibody mediated, cell mediated, or both. Antigens are the components responsible for vaccine function and generally consist of a portion of the pathogenic organism, such as an inactivated or attenuated form of the whole microorganism, or are produced recombinantly from a non-infectious organism engineered to express antigens from a pathogenic species or recombinant protein expressed in genetically engineered cell line. In the case of vaccines derived from recombinant technology, the antigen is expressed in a host cell of an unrelated microorganism and then used for vaccine manufacture. In the case of DNA-based vaccines (currently under development), the vaccine would contain nucleotide sequences (genetic material) that encode microbial antigens.