As in man, the immune system of the mouse can be divided into innate and adaptive components. The innate immune response is evolutionally primitive with functions such as phagocytosis present in protozoa and sponges. Cells have receptors capable of recognizing structural motifs shared by families of infectious agents, and these cells are capable of making an immediate response. The adaptive immune response first appeared in jawed fish, agnathia. In contrast to the innate system, the adaptive system requires time after foreign agent recognition to develop effector mechanisms. The adaptive system incorporates fine specificity for molecular sequences unique to individual infectious agents as well as the capacity for immunological memory. This chapter presents a general overview of molecular and cellular immunology in the mouse. It is important to better understand the ways in which tonic activity of cellular B-cell receptor suppresses receptor editing and to determine if this contributes to the mechanism underlying the all too frequent development of autoreactive B cells in both tissue specific and systemic autoimmune diseases. © 2007 Elsevier Inc. All rights reserved.