Neutrophils engulf and then completely destroy pathogens then die. Dead neutrophils and pathogens form pus at the site of infection.Macrophages engulf the pathogens and then combine the fragments of bacterial or viral protein with a protein complex called the major histocompatibility complex.This then migrates to the cell surface where it displays the fragment on the cell surface.These cells are called antigen-presenting cells (APCs).APCs can be infected body cells or macrophages.Cancer cells also present antigens on their cell membranes.T-Helper cells that have a cell-surface receptor that is complementary to an antigen on an APC. This is called clonal selection.The T-Helper cells will attach to the antigen and become activated.The T-Helper cells will start to divide rapidly by mitosis and form a clone of genetically identical cells. This is called clonal expansion.Some T-Helper cells will differentiate into memory cells, some will stimulate phagocytes to engulf pathogens by phagocytosis some will stimulate B cells to divide and secrete antibodies and other will activate cytotoxic T cells.