As early as the 1970s it had become evident that complement not only played a role in pathogen recognition and elimination but also played a role in B cell biology when it was observed that B cells could bind C3 (see Carroll 2004 for a review). Subsequently, it was recognized that the complement receptors CR1 and CR2 mediate complement-associated B cell functions. These receptors are expressed on B cells and follicular dendritic cells as well as on a subset of T cells. Furthermore, the CR2-CD19-CD81 complex modulates B-cell receptor signaling, thus affecting the amplitude of B cell responses when presented with antigens. CR1 and CR2 also appear to play a role in B cell differentiation, selection, maintenance and elimination of self-reactive B cells (Carroll 2004) .