Immunoblasts are the most immature members of the protective cells involved in an immune response.
Large lymphocytes (12–18 μm in diameter) are probably activated, i.e. immunoblasts, cells with a paler and wider rim of cytoplasm and are often mistaken for monocytes.
They have moderate-to-abundant basophilic cytoplasm and a prominent, centrally located, trapezoid-shaped single nucleolus which often has fine strands of chromatin attached to the nuclear membrane (‘spider legs’).
In some cases, immunoblasts show some morphologic features of plasma cells.
Centroblasts are distinguished from immunoblasts by being large lymphoid cells containing a moderate amount of cytoplasm, round to oval vesicular (i.e. containing small fluid-filled sacs) nuclei, vesicular chromatin, and 2–3 small nucleoli often located adjacent to the nuclear membrane.