An immunoblast is a lymphocyte that has been activated by an antigen, which will further undergo clonal expansion to increase the number of lymphocytes capable of binding to that antigen. Immunoblasts are the most immature members of the protective cells involved in an immune response.Activated B cells may differentiate into memory cells or plasma cells, while activated T cells may differentiate into memory cells or effector cells that aid in the immune response.

The majority of lymphocytes are small and have a thin rim of cytoplasm, 6-9 µm in diameter, about the same size as erythrocytes (diameter 7.5 µm). Large lymphocytes (12-18 µm in diameter) are probably activated, i.e. immunoblasts, with a paler and wider rim of cytoplasm and are often mistaken for monocytes.