Main Difference – Leukocytes vs Lymphocytes
Leukocytes and lymphocytes are found in the blood of vertebrates. Leukocytes are composed of granulocytes and agranulocytes. Three types of granulocytes are found in blood. They are neutrophils, eosinophils and basophils. Granulocytes are involved in the host defense through innate immunity. Lymphocytes are agranulocytes and are involved in the adaptive immunity by producing specific antibodies for a particular pathogen. Lymphocytes also consist of three types: T lymphocytes, B lymphocytes and a null group, containing natural killer cells and cytotoxic cells. The antigens which are presented by granulocytes are identified by T lymphocytes, activating B lymphocytes to produce the specific antibodies. The main difference between leukocytes and lymphocytes is that leukocytes are all the white blood cells in the blood whereas lymphocytes are one type of blood cells, involved in the adaptive immunity of vertebrates.
This article explains,
1. What are Leukocytes
– Characteristics, Structure, Function
2. What are Lymphocytes
– Characteristics, Structure, Function
3. What is the difference between Leukocytes and Lymphocytes
What are Leukocytes
Leukocytes are the only type of nucleated cells found in blood, involved in the host defense by destroying pathogens which invade the body of vertebrates. They are generally called white blood cells. Leukocytes can be divided into two groups, depending on the presence of granules in their cytoplasm: granulocytes and agranulocytes. Three types of granulocytes can be found in the blood: neutrophils, eosinophils and basophils. Each of them is different in their shapes of the nuclei as well as the functions in the body. The process of the formation of leukocytes is called hematopoiesis. During hematopoiesis, leukocytes are differentiated from the stem cells of myeloblast, lymphoblast and monoblast.
Figure 1: Hematopoiesis
Neutrophils
Neutrophils are professional phagocytes, destroying pathogens like bacteria through phagocytosis. They contain a poly-lobed nucleus, which usually consists of 2-5 lobes. The diameter of neutrophils is 8.85 µm. Neutrophils are the most abundant type of leukocytes. 40-75% of white blood cells are neutrophils. Normal range for eosinophils is 1,500-8,000 neutrophils per mm-3. The lifespan of neutrophils is 5-90 hours in circulation. The granules of neutrophils contain lysozyme, phopholipase A2, acid hydrolases, myeloperoxidase, elastase, serine proteases, cathepsin G, proteinase 3, proteoglycans, defensins and bacterial permeability increasing protein. Neutrophils are one of the first cells migrating to the site of inflammation, responding to cytokines released by inflammatory cells. The process of migration of neutrophils into the inflammation site is called chemotaxis. Activated neutrophils produce neutrophil extracellular traps (NETs).
Eosinophils
Eosinophils provide the defense against parasites like helminth. The nucleus is two-lobed in eosinophils. The diameter of eosinophils is 12-17 µm. 1-6% of white blood cells are eosinophils. Normal range for eosinophils is 0-450 eosinophils per mm-3. Cytotoxicity is the process eosinophils provide to defend against the common hypersensitivity reactions. The cytotoxicity is mediated by the cationic proteins included in the cytoplasmic granules. Granules contain histamines, RNase, DNase, eosinophil peroxidase, palsminogen, lipase and major basic proteins. Basophils and mast cells also contribute while responding to allergic reactions. Eosinophils are capable of migrating to tissues as well. Thus, they are found in thymus, spleen, ovary, uterus, lymph nodes and lower gastrointestinal tract. The lifespan of eosinophils is 8-12 hours in circulation. In tissues, it is 8-12 days. By the activation of eosinophils, cytokines like TNF alpha and interleukins, growth factors like TGF beta and VEGF and some other species are produced.
Basophils
Basophils along with the mast cells produce cytokines against parasites. The nucleus is bean-shaped in basophils. The diameter of basophils is 10-14 µm. Basophils are the least common type of granulocytes in blood. 0.5-1% of white blood cells are basophils. Normal range for basophils is 0-300 basophils mm-3. The lifespan of basophils is 60-70 hours. These cytokines provide defense against allergic inflammation. Granules contain histamine, proteolytic enzymes like elastase and lysophospholipase and proteoglycans like heparin and chondroitin. The histamine and heparin in the granules prevent blood clotting while circulating. Basophils play a role in providing defense against viral infections as well. Leukotrienes and some interleukins are secreted by activated basophils.
Monocytes
Monocytes are the only agranulocytes found in leukocytes other than lymphocytes. They are involved in the intercellular killing of pathogens. They possess an immediate response prior to the entrance of other WBCs into the infected area. The migration into the inflammatory tissue lets the monocytes to differentiate into macrophages, which are the kind of professional phagocytes. Macrophages also present antigens to T lymphocytes, promoting the generation of adaptive immune responses.
The other type of leukocytes are lymphocytes, which is described below in the article.
What are Lymphocytes
Lymphocytes are the last type of leukocytes, involved mainly in the adaptive immunity by producing specific antibodies to a particular pathogen during host defense. During hematopoiesis, lymphocytes are differentiated from lymphoblastic stem cells. The three main types of lymphocytes are T lymphocytes, B lymphocytes and natural killer cells. T lymphocytes are involved in the humoral immunity and the differentiated plasma cells from B lymphocytes secrete specific antibodies for a particular pathogen.
Mature T lymphocytes express T cell receptors (TcRs), which are specific to a particular antigen. CD3 molecules are expressed on the membrane, associating withTcR. one type of accessory molecules, either CD4 or CD8 are expressed on the membrane of T cells as well.TcR/CD3 is capable of identifying antigens, presented on MHC complex on the infected cells. Three types of T cells are there: T helper cells, Tcytotixic cells andT suppressor cells. T helper cells affect B lymphocytes by activating them to produce specific antigens to a particular pathogen. T cytotoxic cells are cytotoxic against tumor cells, while presenting the antigens of pathogens along with MHC class I molecules. T and B cell responses are suppressed by T suppressor cells.
Figure 2: T cell-dependent B cell activation
B lymphocytes are activated by T cells and the antibody, IgM is produced as the primary immunization, which can be identified in the serum after 3-5 days of infection. The level of IgM peaks in 10 days after the infection. B cells also present antigens of digested pathogens along with MHC II complexes. A portion of B cells become memory B cells, storing the memory of invaded pathogens for long periods of time. Natural killer (NK) cells are granular lymphocytes, which non-specifically phagocytize infected cells by viruses and tumor cells. The digestion of these cells by NK cells secretes IFN-gamma and IL-2. NK cells express a surface receptor CD16. Activated NK cells secrete INF-alpha and TNF-gamma as well.
Figure 3: Natural killer cell
Difference Between Leukocytes and Lymphocytes
Correlation
Leukocytes: Leukocytes refer to all the white blood cells in the blood.
Lymphocytes: Lymphocytes are one type of white blood cells in the blood, involved mainly in the adaptive immunity during the host defense.
Composition
Leukocytes: Leukocytes are composed of both granulocytes and agranulocytes.
Lymphocytes: Lymphocytes are mainly composed of only agranulocytes.
Types
Leukocytes: Leukocytes consist of neutrophils, eosinophils, basophils and lymphocytes.
Lymphocytes: Lymphocytes are composed of T lymphocytes, B lymphocytes, and a null group, containing natural killer cells and cytotoxic cells.
Production
Leukocytes: Leukocytes are produced either in myeloid stem cells or lymphoid progenitor cells.
Lymphocytes: Lymphocytes are produced in lymphoid progenitor cells.
Role in Host Defense
Leukocytes: Leukocytes are involved in both innate and adaptive immunity during host defense.
Lymphocytes: Lymphocytes are mainly involved in the adaptive immunity during host defense.
Conclusion
Leukocytes are the white blood cells found in blood. Five major types of leukocytes are found in blood. They are neutrophils, eosinophils, basophils monocytes and lymphocytes. Neutrophils, eosinophils and basophils are granulocytes, containing various contents in their granules. They are mainly involved in the innate immunity, where the defense system of the host generate the same immune response non-specifically for all pathogens. These granulocytes destroy pathogens like bacteria, viruses and parasites by pagocytosis. While destroying pathogens, they present antigens of those destroyed pathogens on their cell membrane. Monocytes are type of leukocytes, which lack granules. But monocytes serve as professional phagocytes by differentiating into macrophages inside the inflammatory tissues. The resultant antigens are recognized by T helper cells, letting the B lymphocytes to generate specific antibodies for a particular antigen. Therefore, lymphocytes are involved in the adaptive immunity of the host defense mechanisms. Natural killer cells are a type of circulating lymphocytes, which phagocytize viral infected cells and tumor cells. They are type of granulocytes. However, the main difference between leukocytes and lymphocytes is the type of immunity they generate during host defense.
Reference:
1. Goldman, Armond S. “Immunology Overview.” Medical Microbiology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1996. Web. 05 Apr. 2017.
Image Courtesy:
1. “Illu blood cell lineage” (Public Domain) via Commons Wikimedia
2. “T-dependent B cell activation” By Altaileopard – Own work (Public Domain) via Commons Wikimedia
3. “Human Natural Killer Cell” by NIAID (CC BY 2.0) via Flickr
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