Characterization of postpartum lobule involution in human breast tissue. A: H&E-stained breast biopsy tissues from developmental stages representative of nulliparous (N), pregnant (P), lactation (L), involution (I), and regressed (R); scale bar represents 100 microns. Tissue sections Stained by IHC for: B: CD45, scale bar represents 100 microns. C: CD68, scale bar represents 100 microns. D: Quantification of CD45 staining as described in Materials and Methods section. Number (n) of cases for each developmental stage: nulliparous n = 5, pregnant n = 8, lactation n = 11, involution n = 8, regressed n = 10, * different from involution, P < 0.0001, Tukey–Kramer Multiple Comparisons Test. Black bars indicate average CD45 stain signal values for each developmental stage. E: Quantification of CD68 staining. Number (n) of cases for each developmental stage: nulliparous n = 6, pregnant n = 9, lactation n = 9, involution n = 8, regressed n = 8, * different from involution, P < 0.0001, Tukey–Kramer Multiple Comparisons Test. Black bars indicate average CD68 stain signal values for each developmental stage. F: Serial sections of human involuting breast biopsy tissue stained by IHC for CD45 (monocytes), CK18 (epithelial cells), and CD68 (macrophages) to reveal multiple cell populations present in human breast tissue, arrowheads indicate cells that are CD45- and CD68-positive but CK18-negative, arrows indicate cells that are either CD45-positive or CD68-negative and CK18-negative, scale bar represents 50 microns. G: Dual fluorescent staining of human involuting breast biopsy tissue using E-cadherin (green) to identify mammary epithelial cells and CD68 (red) to identify macrophages (arrow). Nuclei are stained blue with DAPI, the dotted line indicates the acinar lumen, scale bar represents 10 microns. H: Human involuting breast biopsy tissue stained by IHC for Macrophage Mannose Receptor (arrow) and M2 cytokine IL-13 (arrow), scale bar represents 50 microns. I: Involution (Inv) and nulliparous (N) cases from CD45 quantitation separated based on presence (+), or absence (−), of cancer in tissues examined.