The vaccine contains parts of the pathogen that are recognised by the immune system, these are called antigens. The antigen is recognised by innate immune cells such as macrophages that will engulf the pathogen and express the antigens on its surface. This will then prime cells of the adaptive immune system that have a specific receptors for the antigen, this includes T and B cells. T cell stimulation is required to activate B cells. Once activated B cells will secrete a soluble form of their receptor called antibodies in large amounts. Antibody secreting B cells are called plasma cells.