Active vs passive humoral immunity, naturally and artificially acquired immunity, as well as primary and secondary immune responses. Active humoral immunity: b cells encounter and produce antibodies against them. Naturally acquired: exposure to disease, bacterial or viral infection. Occurs when b cells encounter antigens and produce specific antibodies against them. Two types of active humoral immunity: naturally acquired : formed in response to actual bacterial or viral infection, artificially acquired : formed in response to vaccine of dead or attenuated pathogens. Provide antigenic determinants that are immunogenic and reactive. Occurs when ready-made antibodies are introduced into body. Two types of passive humoral immunity: naturally acquired: antibodies delivered to fetus via placenta or to infant through milk, artificially acquired: injection of serum, such as gamma globulin. T cells provide defense against intracellular antigens (e. g. virus infected cells), tumor or abnormal cells, graft tissues. Some t cells directly kill cells; others release chemicals that regulate immune response.