The human immune system is composed of two major parts, the adaptive immune system and the innate immune system. The initial reaction to a pathogen involves the innate system, a non-specific response that involves inflammation, leukocytes, and the complement system. The adaptive immune system response is carried out by B cell and T cell lymphocytes. B cells are activated to secrete antibodies to the pathogen, present antigen, and secrete cytokines and various T cells kill target cells, activate macrophages, and stimulate B cells to produce antibodies. The adaptive immune system also creates immunological memory through memory B and T cells that provide enhanced protection to future infection.