Type AB Thymoma

Mixture of variable proportions of lymphocyte-poor and lymphocyte rich areas
- May be discrete areas or intermixed
Lymphocyte poor areas have features of Type A thymoma
- Bland epithelial cells with few lymphocytes
Lymphocyte rich areas differ from types B1, B2 and B3
- Epithelial cells are essentially the same as those of Type A
  - Spindled to oval cells
  - Nuclei are bland, small, round to oval
    - Dispersed chromatin
    - Inconspicuous nuclei
Type AB thymoma is not simply a mixture of Type A and Type B patterns
- It is better considered as a Type A thymoma with lymphocyte rich areas

Epithelial cells (in both components)
- Keratin positive
- EMA negative to focally positive
- CD20 focally positive in most cases
- CD5 negative
Lymphocytes
- Variable populations of mature and immature (CD1a, CD99, TdT) T phenotypes
Few B cells

Type AB Thymoma	Micronodular Thymoma with Lymphoid Stroma
Lymphocyte rich areas contain scattered epithelial cells	Lymphocyte rich areas lack epithelial cells
Lymphocyte rich areas composed of T cells	Lymphocyte rich areas contain many B cells, frequently with germinal centers

Type AB Thymoma	Type B2 Thymoma
Epithelial cells have small oval to spindled nuclei with dispersed chromatin and inconspicuous nucleoli	Epithelial cells have vesicular nuclei with prominent nucleoli
Areas of Type A are usually present	Type A areas not present

Both may be composed of an essentially even mixture of epithelial cells and lymphocytes

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