place the following steps in the correct order to represent the stages in which antigens are formed by the immune system. a)The pathogen is ingested by the macrophage and fragments into pieces. b)A fragment of the pathogen, called an antigen, is attached to the surface of the macrophage. c)The expression of this antigen on the macrophage, which is now called an antigen- presenting cell, alerts the remainder of the immune system. d)A pathogen is detected by the immune system.