TEM image, 5,000X (photo courtesy of V. P. Saprykin). Lymphocytes that circulate in the blood stream have rounded nuclei (sometimes with some invaginations) and nucleoli. The ratio euchromatin to heterochromatin may differ in nuclei of different lymphocytes. The cytoplasmic volume is not considerable; there are generally free polyribosomes #1, #2. Cisterns of endoplasmic reticulum and mitochondria are seldom found there.

In an antigen stimulation state, lymphocytes undergo blast transformation where their cytoplasmic volume and number of organelles increase as a result of differentiation into effector cells for an immune response.