Released into the bloodstream, antibodies lock onto matching antigens. These antigen-antibody complexes are soon eliminated, either by the complement cascade or by the liver and the spleen. T cells are mobilized when they encounter a cell such as a macrophage or a B cell that has digested an antigen and is displaying antigen fragments bound to its marker molecules. Lymphokines help the T cells to mature. The T cell, alerted and activated, secretes lymphokines.
Some lymphokines spur the growth of more T cells.
Some T cells become killer cells and track down body cells infected by viruses. Yet other lymphokines direct the recruits once they arrive on the scene. The next time that an individual meets up with the same antigen, the immune system is set to demolish it. An immune response can be sparked not only by infection but also by immunization with vaccines. Infants, who are born with weak immune responses, are protected for the first few months of life by antibodies they received from their mothers before birth.