B lymphocyte The pathogen that invades the organism produces antigens which are then taken by the B lymphocytes and macrophage cells The macrophage cells and the B lymphocytes process the antigens and link them to a protein (MHC), which presents the processed antigen on the surface of the cell. A T helper is attached to the processed antigen on the cell and becomes activated, making it capable of interacting with B lymphocytes. The T helper attaches to the MHC proteins with antigens on the surface of the B lymphocyte. The B lymphocyte is activated to divide by mitosis to give a clone of plasma cells. The cloned plasma cells produce antibodies that fit antigens on the pathogen’s surface. The antibodies attach to the antigens on the pathogen, destroying it. Some of the plasma develop into memory cells which survive for long time. So, for next infections from the same pathogens. The process only starts from memory cells. T Lymphocytes Viruses invade body cells and are taken by macrophage cells. The macrophage cells and the B lymphocytes process the antigens and link them to a protein (MHC), which presents the processed antigen on the surface of the cell. A T helper cell attaches to the antigen on the surface of the macrophage cells and is stimulated to divide by mitosis. Some of the new T helper cells convert into memory cells which can survive for longer time and can react rapidly. Other T helper cells produce cytokines which stimulates B lymphocytes and macrophage cells. The cytokines also cause T cytotoxic cells to divide by mitosis. Some of the cells from memory cells that survive and respond rapidly to a new infection. Other T cytotoxic cells attach to any body cell presenting the viral antigen. The attached T cytotoxic cells produce perforins to make holes in the cell membrane and so destroy the cell, along with the viruses it contains.