Lymphocytes

Lymphocytes
Appearance:

Lymphocytes have a round to oval nucleus and are approximately the size of erythrocytes. Cytoplasm is grey-blue. The volume of cytoplasm varies widely. As a result, cell size varies between 9 and 20 µm. Large and small lymphocytes are distinguished by the volume of cytoplasm. The difference in size is an expression of different states of activation.
Normal range:
The normal lymphocyte count lies between and x 109/L. Therefore, they are the second most frequent leukocyte in the blood film after neutrophilic granulocytes. If the lymphocyte count is low, lymphopenia is present. If it is increased, then lymphocytosis is present. Morphologically, B and T lymphocytes cannot be distinguished from one another.
Function:
Lymphocytes participate in cellular and humoral defense against  infection.
Special form: Large granular lymphocytes

A special lymphocyte form is the large granular lymphocytes, or LGL for short. These cells have a large volume of cytoplasm with clear azure granules. They constitute 5-10% of all lymphocytes. They correspond to either a subpopulation of T-cells or natural killer cells (NK cells).
Clincal Significance:
Lymphocytosis may occur primarily in the case of lymphoproliferative syndromes or secondarily, i.e. reactively, in the case of infections (e.g. pertussis, mononucleosis, and other viral infections), stress (e.g. myocardial infarct) and chronic inflammatory diseases.
Lymphopenia can occur in cases of infections (e.g. HIV, tuberculosis), radiotherapy and systemic diseases as well as  during immune suppressive therapy.