As a subgroup of leukocytes (white blood cells), lymphocytes play a central role in the immune defense against foreign substances, especially infectious agents, as well as pathogenically altered cells of the human organism such as tumor cells. An increased or decreased concentration of lymphocytes in the blood usually indicates a disease.
What are lymphocytes?
Lymphocytes are components of the blood. They belong to the natural “killer cells” as well as to the white blood cells, the leukocytes. In the image, lymphocytes destroy cancer cells. White: lymphocytes, green: cancer cells. Click to enlarge. Lymphocytes are the smallest representatives of leukocytes and are the most important carriers of the adaptive (acquired) immune system of the human organism. In addition to blood plasma, human blood is composed of approximately 45 percent blood cells, which are divided into leukocytes (white blood cells), erythrocytes (red blood cells), and platelets (thrombocytes). In general, a differentiation is made between B and T lymphocytes and NK cells. In an adult human, 1000-2900 lymphocytes per µl of blood or 17-47 percent of the white blood cell percentage is considered normal. The largest proportion of lymphocytes does not circulate in the bloodstream, but is found in the bone marrow and in the organs of the lymphatic system (thymus, tonsils, spleen, Peyer’s plaques of the intestinal tract, lymph nodes). An increased or decreased lymphocyte count may indicate various diseases.
Medical and health functions, roles, and meanings.
Depending on the mode of maturation, lymphocytes are divided into B and T lymphocytes and NK cells. The B cells (derived from bone marrow or the bursa fabricii in birds, where B lymphocytes were first discovered), which begin their maturation process in the bone marrow, are capable of producing and secreting antibodies (defensive substances) that specifically neutralize soluble antigens (including bacteria, released toxins) classified as foreign to the body. For this purpose, the inactive B lymphocytes circulate in the lymphatic system or bloodstream and are activated as soon as an antigen docks with the surface immunoglobulins, the antigen receptors of the B cells. The B cell takes up the antigen, disassembles it, and expresses it as a protein complex that is identified by the T helper cells (subset of T lymphocytes). In addition, the T helper cells synthesize cytokines that activate B lymphocytes, which subsequently proliferate (divide) in the lymph nodes or spleen. In addition, a smaller proportion of B lymphocytes differentiate into long-lived memory B cells that store antigen information to ensure a timely and effective immune response upon further contact with the specific antigen. The T-lymphocytes, which mature in the thymus, act as an ordering and control body to identify foreign particles (including antigens such as viruses, intracellular bacteria, cells modified by mutations) that can exert a damaging effect on the organism and prepare the appropriate immune defense cells to ensure a rapid and targeted defense against the identified pathogens. NK cells, or natural killer cells, primarily recognize modified endogenous cells such as virus-infected cells or tumor cells and trigger apoptosis, or programmed cell death, in them.
Diseases, ailments, and disorders
A pathologic increase (lymphocytosis) or decrease (lymphopenia) in the number of lymphocytes can be attributed to a variety of causes. A decreased or increased number of lymphocytes detected on a differential blood count with determination of all subtypes of leukocytes in the blood indicates a possible disease. Thus, an increase in lymphocytes and leukocytes in the blood usually correlates with inflammation or infection. Viral droplet infections (including influenza, measles, rubella, mumps, chickenpox), contact and smear infections (herpes simplex, diarrhea, hepatitis A and E, polio, Ebola, ]]yellow fever]], HIV, cytomegalovirus), bacterial infections (brucellosis, tuberculosis, typhoid, pertussis, resp. whooping cough) as well as various tumor diseases (leukemia, lymphoma) are associated with an increased concentration of lymphocytes in the blood.Similarly, hyperthyroidism (hyperthyroidism), Guillain-Barré syndrome (progressive impairment of the nervous system) or sarcoidosis or Boeck’s disease, an inflammatory disease and granulomatosis (focal accumulation of granulomas) that predominantly affects the lungs, can cause elevated lymphocyte levels. In contrast, chemotherapy and/or radiotherapy, cortisone therapy, cytostatic therapy, or treatment with immunosuppressants, as well as an increased cortisol concentration (hypercortisolism), for example, as a result of Cushing’s syndrome, can cause a lowered lymphocyte level in the blood. In addition, autoimmune diseases (for example, a pronounced systemic lupus erythematosus or myasthenia gravis), various cancers (including Hodgkin’s disease or lymphoma), uremia (urine poisoning in the final stage of renal failure) and AIDS can cause a decreased lymphocyte concentration in the blood.