B-lymphocyte response during involution. (a) Pearson correlation clustering of immunoglobulin genes. The colours (see colour bar) represent a relative measure for the expression level, with 1 representing the median. Forty-nine immunoglobulin genes that were upregulated during involution were clustered by using the Pearson correlation clustering function from GeneSpring (Silicon Genetics); their average expression (red line) and standard deviations (blue lines) are shown in the graph on the right (DMT®; Affymetrix). Note that the y-axis represents only an arbitrary level of expression. Abbreviations: vir, virgin; preg, pregnancy; lac, lactation; inv, involution. (b) Cell count of plasma cells and B220+ B lymphocyes present in the mammary tissue during involution. Tissue sections were stained either with methyl green and pyronin or with an antibody against CD45R, and plasma cells and B lymphocytes were counted. Cell counts were taken from 20 high-power fields from each section in triplicate from three individual mice.