| Lymphocytes originate from a marrow stem cell and mature in lymph nodes, spleen, and associated peripheral lymphoid tissues. Mature lymphocytes consist of 2 subpopulations, B cells and T cells.
B cells (B for bone marrow or bursa equivalent) are the precursors of plasma cells and produce antibodies for humoral immunity.
T cells (T for thymus) engage in cellular immunity (eg, histocompatibility and delayed-type hypersensitivity). A lymphocyte in tissue may return to the vascular bed and recirculate. Some lymphocytes are long-lived compared with other WBC and may survive weeks to years. |
| A peripheral
lymphocytosis has many possible causes, including a physiologic (epinephrine) lymphocytosis, immune stimulation, and lymphocytic leukemia. Immune (antigenic) stimulation is associated with chronic inflammation and characterized by the presence of reactive (immunologically stimulated) lymphocytes. These lymphocytes have a more basophilic and slightly more abundant cytoplasm due to increased protein synthesis. Reactive lymphocytes may occur in any disease that causes
moderate to marked systemic immunostimulation. Lymphocyte counts of up to 17,000/µL, with some cells containing intracytoplasmic azurophilic granules, have been reported in dogs infected with
Ehrlichia
canis
. Persistent lymphocytosis in cattle is a B-cell hyperplasia, with lymphocyte counts ranging from 7,000-15,000/µL. |
|
Lymphopenia is a common leukogram abnormality. It is most commonly associated with endogenous (stress) or exogenous corticosteroids and is the result of redistribution or lysis of lymphocytes. Lymphopenia also occurs due to other causes, such as extravasation of lymph (eg, lymphangiectasia, chylous effusion), impaired lymphopoiesis, some viral infections, and hereditary immunodeficiency diseases (eg, combined immunodeficiency disease of Arabian foals). |
