The normal or benign lymph node is composed of a collection of follicles and interfollicular areas surrounded by sinuses (spaces mostly filled with histiocytes), vessels, and sometimes fat. Lymph flows from the subcapsular sinuses, through the medullary sinuses in the lymph node, and out the hilum. The follicles represent areas of maturing B cells (CD20⁺), whereas the interfollicular areas are mostly mature T cells (CD3⁺).

Follicles begin as primary follicles or aggregates of antigen-naïve B cells. As they mature into secondary follicles, they acquire germinal centers, which are visible as targetoid nodules within the follicles (Figure 21.1). The dark outer rim of the follicle is the displaced remains of the primary follicle and is called the mantle zone, still composed of antigen-naïve B cells. Once exposed to antigen, the B cells move to the germinal center and become centroblasts, large cells with primitive-looking nuclei. From there they either mature into centrocytes or die through apoptosis . Finally, B cells leave the germinal center genetically altered to circulate as memory B cells, which may ultimately differentiate into plasma cells if they meet their antigen.

Other normal germinal center components include the supporting follicular dendritic cells and tingible body macrophages, which clean up the apoptotic debris. These macrophages appear as relatively clear cells within the germinal center, with visible “dust specks” in the cytoplasm (Figure 21.2). Germinal centers may be found in any hotbed of lymphocyte activity outside the lymph nodes, but the morphology and staining pattern are preserved.

The paracortex, or interfollicular area, may occupy most of the lymph node in some cases. This absence of obvious follicles is not necessarily a reason for concern. The benign paracortex should have a mottled appearance due to the scattered pale histiocytes among the T cells.

The word lymphoma means a malignancy of the lymphoid system and usually implies a solid tumor mass, whether it is in the lymph nodes or an extranodal site. However, remember that many of the lymphomas can have an associated liquid phase, or leukemia, in which the neoplastic cells invade the bone marrow and enter the peripheral blood (Table 21.1). Among the myeloid leukemias, discussed in Chapter 20, solid tumor disease is uncommon but exists (such as the chloroma or granulocytic sarcoma).

This chapter covers several major categories of lymphomas:

Many of the lymphomas can be placed into categories by nuclear morphology, and learning to recognize the “look” of each group is important, although actual diagnosis is primarily by flow, immunohistochemistry (IHC), and molecular studies. The lymphoblastic lymphomas have immature chromatin, which means the texture is very fine grained, with small nucleoli and an indistinct nuclear membrane, much like the myeloblasts in acute myeloid leukemia. On H&E stain, they may be mistaken for small cell carcinoma or a primitive sarcoma. The low-grade neoplasms resemble normal lymphocytes , with small condensed nuclei. The high-grade lymphomas show very carcinoma-like nuclei: they are large (compared with lymphocytes) and pleomorphic, with prominent nucleoli and coarse nuclear membranes. Hodgkin lymphomas are the hardest to identify, usually, as the diagnostic cells (Reed–Sternberg cells and variants) may be few and far between. However, Reed–Sternberg cells do resemble high-grade nuclei in their chromatin pattern.

Recognizing a lymphoma in an extranodal site, especially a tumor of unknown origin, takes practice. Clues to lymphoma include a relatively homogeneous, sheet-like growth of malignant cells; a lack of cell-to-cell cohesiveness or architecture; nuclei that are highly irregular in shape or contour; and an accentuation of cell density around vessels (especially in the brain, Figure 21.3). Most should stain for CD45, the common leukocyte antigen, or for specific B or T markers. Positive staining for melanoma markers or cytokeratins rules out lymphoma. Sarcoma markers should be used with caution, though, as many of the familiar stains (CD117, CD34, etc.) also stain hematopoietic elements.

DLBCL is not usually mistaken for a benign entity; the nuclei are too abnormal. However, it may be mistaken for other types of malignancy, especially given its tendency to crop up in extranodal sites. As described earlier, the nuclei are very irregular in contour, with cleared-out or vesicular chromatin leaving a prominent nucleolus and thick nuclear rim (Figure 21.4). Folded, or cleaved, nuclei are common. The cells may have more cytoplasm than lymphocytes and therefore a lower N/C ratio.

Although DLBCL was formerly a single category in the World Health Organization classification, the splitters are gaining on it. One division is between those DLBCLs that are of germinal center cell origin, such as a follicular lymphoma gone bad, and those of activated B cell (or non-germinal center) origin. The latter have the worse prognosis; the distinction is through an IHC staining algorithm . Other subtypes of DLBCL are the so-called double- and triple-hit lymphomas , which are identified through their rearrangements of the MYC, BCL2, and/or BCL6 genes. Such lymphomas are now called high-grade B cell lymphomas (HGBL) , with the genetic rearrangements specified. Additional variants of DLBLC are identified based on unique molecular events and/or viral associations; this list grows longer each year.

Burkitt lymphoma is a distinct type of high-grade B cell lymphoma that is identified by its high mitotic rate (Ki67 index of nearly 100%) and its population of medium-sized, densely packed lymphocytes with intermixed macrophages and apoptotic bodies (Figure 21.5). The scattered macrophages on a blue background give rise to the “starry sky” analogy. The individual cells are atypical enough that they would not be mistaken for normal mature lymphocytes, however. Burkitt lymphoma is EBV-associated in some cases, and most cases involve a MYC translocation.

Follicular lymphoma is the second most common non-Hodgkin lymphoma. Together DLBCL and follicular lymphoma account for over half of non-Hodgkin lymphomas. Follicular lymphoma is defined by a translocation in which BCL2 (an antiapoptotic factor) is abnormally upregulated. BCL2 usually turns off in germinal centers, making the centroblasts and centrocytes susceptible to apoptosis. Abnormal retention of BCL2 leads to cells that do not die, more or less, hence the malignancy. Follicular lymphoma appears as a nodular proliferation of back-to-back neoplastic follicles that fill the lymph node (Figures 21.6 and 21.7). Within these follicles are a mixture of neoplastic centrocytes (smaller) and centroblasts (larger); the relative proportion determines the grade of the lymphoma. Follicular lymphoma can also have areas of diffuse growth (the opposite of nodular), can spread to the marrow, and can transform to DLBCL. When circulating as a leukemia, the folded (cleaved) centrocyte nucleus has been compared to a “baby’s butt.”