Lymphocytes

These are the main types of specific immune response cells, leukocytes of mammals. Normally, lymphocytes make up 20–40% of white blood cells (leukocytes), constantly circulate in the blood and lymph, migrate to tissues and lymphoid organs. They determine the diversity, specificity, and immune memory of the immune response. There are 10¹⁰–10¹² lymphocytes in the human body. They are round, 7–12 µm in diameter, non-phagocytic cells. There are three types of lymphocytes: T lymphocytes, B lymphocytes, and natural killers (NK cells). NK cells mature in the bone marrow, they make up about 10% of circulating lymphocytes, belong to granulocytes. They are found in the blood and lymphoid tissue (especially in the spleen). They are an innate part of the immune system, protecting the body from tumor and virus-infected cells. These cells are activated by interferons. Activated NK cells release cytotoxic (cell-killing) granules that destroy altered cells. T and B lymphocytes are the main components of acquired immune response. They belong to agranulocytes. T lymphocytes are responsible for cellular immunity, i.e. they recognize antigens on infected or tumor cells, while B lymphocytes are responsible for humoral immunity, i.e. B lymphocytes and their secreted antibodies recognize circulating antigens (pathogenic microorganisms or toxins). T lymphocyte precursors are in the bone marrow, but they mature in the thymus gland (T – Thymus), and later differentiate into helper T cells (T_H or CD4⁺ T lymphocytes, because their membranes contain CD4 molecules) and cytotoxic T cells (T_C or CD8⁺ T lymphocytes). The T_H to T_C ratio is approximately 2:1. T_H, when responding to certain pathogens, secrete cytokines that are important for the immune response. T_C release granules containing enzymes that cause the death of cells infected with pathogens or tumors. B lymphocytes mature in the bone marrow. The antigen receptors on the surface of B lymphocytes are inserted into the plasma membrane and exhibit high specificity for antibodies. After activation, B lymphocytes differentiate into plasma cells, which produce soluble protein antibodies (immunoglobulins) and memory cells. T and B lymphocyte memory cells “remember” the antigen and, upon its repeated entry into the body, trigger a rapid and strong response. Lymphocytes finish maturing in lymph nodes, where they become effective cells, re-enter the lymph and blood, and are transported to tissues.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė