Importance of Immunohistochemistry in the Evaluation of follicular lymphoid lesion

Before discussing the utility of particular immunostains, it is important to review and understand some basics about the normal lymphoid follicle, in order to interpret the immunostains appropriately.
The difference between a primary follicle and a secondary follicle:
A primary or "resting" follicle: - a collection of B-cells in the cortex of the lymph node that has not been antigenically stimulated, and because of this, it does not have a germinal center.
A secondary follicle:- Once a primary follicle has been antigenically simulated, it acquires a germinal center, and at this point is referred to as secondary follicle.
Differential diagnosis of lymphoid lesion with follicular structures or nodules, :-
1) Resting (primary) follicles,
2) Secondary follicles (as a reflection of reactive follicular hyperplasia),
3) Follicular lymphoma,
4) Mantle cell lymphoma with a nodular growth pattern.
5) B cell SLL/CLL with prominent pseudofollicular proliferation centers.

Many times the distinction of non-neoplastic lymphoid follicles vs. neoplastic lymphoid follicles can be readily made on standard H&E morphology alone.
As we all know however, we see cases where this distinction is challenging, and in other situations we are faced with minuscule or suboptimal material.By understanding the expected phenotype of the lymphoid cells within each of the various types of lymphoid follicles, one can frequently render a confident diagnosis, even in the face of suboptimal material or a minimal biopsy sample.

The lymphocytes in primary (resting) follicle express B cell markers (such as CD20, CD79a, or Pax-5) and BCL2.CD23 may be positive or negative, but they lack BCL6,CD10, CD5, and cyclin D1, and they have a low Ki-67 proliferative fraction.
In contrast, in the germinal center of a secondary follicle, the lymphoid cells lack BCL2, CD5,CD23, and cyclin D1, express both BCL6 and CD10, and have a very high proliferative fraction, approaching 100%.
In follicular lymphoma, the neoplastic cells express B-cell markers, BCL2 and BCL6, and lack CD5, CD23 and cyclin D1, with a variable Ki-67 proliferative fraction. Most but not all follicular lymphomas express CD10, and some grade 3 follicular lymphoma lack BCL2.
In mantle cell lymphoma, the lymphocytes express B-cell markers, BCL2, CD5, and cyclin D1, typically lack BCL6 and CD23.
And finally, in B-cell small lymphocytic lymphoma/CLL, the neoplastic cells express B-cell markers,CD5, CD23, and BCL2, but lack BCL6, cyclin D1, and CD10.

Caveats for immuno interpretation:
Since some of these follicular or nodular structures to some extent consist of mixed B and T-cell populations, there are often minor populations of cells staining for the markers above that are listed as lacking, and for that reason in some cases it is easiest to interpret the immunostain results on low-power.
For example, in the case of primary (resting) follicles, there are often a small number of background cells that may express BCL6, CD10, and CD5. It is always a good idea to compare the extent of reactivity of the markers discussed with the extent of reactivity of the associated B cell markers. Finally, it must be kept in mind that CD23 stains a subset of the follicular dendritic cells (FDC) that may be present in these conditions, so care must be taken to not misinterpret CD23 reactivity of FDC as reactivity of the lymphocytes. Finally, it is worthwhile to note that some follicular lymphomas contain impressive numbers of non-neoplastic T-cells. Another point that must be made is this: "tumors do not read textbooks". As such, not all lymphoid proliferations will neatly fit into the expected patterns of reactivity discussed
above. For example, we have seen clear-cut cases of both follicular lymphoma and mantle cell lymphoma that have expressed strong CD23, a few mantle cell lymphomas that have expressed BCL6, and we have even seen a rare case of follicular lymphoma that expressed CD5.

Click on the images to enlarge them.

References:
1) CD10 and BCL-6 expression in paraffin sections of normal lymphoid tissue and B-cell lymphomas.Dogan A, Bagdi E, Munson P. Am J Surg Pathol. 2000 Jun;24(6):846-52.
2) Abnormal expression of bcl-10 protein in extranodal marginal zone B cell lymphoma of mucosa-associated lymphoid tissue lymphoma typeLi BZ, Zhou XY, Ye HT, Yang WT, Fan YZ, Lu HF, Shi DR. Zhonghua Bing Li Xue Za Zhi. 2007 Dec;36(12):819-24. Chinese.
3) Propath focus –Immunohistochemistry in the Evaluation of Follicular or Nodular Lymphoid Lesions, R. T. Miller, M.D