Activated plasma cell, coloured transmission electron micrograph (TEM). Plasma cells are derived from stimulated B lymphocytes (white blood cells). They are antibody factories programmed to produce copious quantities of one type of antibody. This activated plasma cell is producing IgG antibodies, seen in dilated endoplasmic reticulum cavities. Immunoglobulin G (IgG) is the most abundant immunoglobulin antibody, and is found in all body fluids. They bind to bacterial or viral proteins, marking the pathogens for destruction by other immune system cells called phagocytes.