Lymphocytes are a type of white blood cells present in the blood and lymph of our body. The main function of lymphocytes is to produce antibodies and protect against viruses, bacteria and toxins. These lymphocytes can be classified into B lymphocytes, T lymphocytes and natural killer cells. Here, let’s discuss the structure and functions of these lymphocytes.
Table of Content
- Lymphocytes – Meaning and Definition
- Lymphocytes Types
- B Lymphocytes
- T Lymphocytes
- Natural Killer Cells
- Lymphocytes Function
- Frequently Asked Questions
Lymphocytes – Meaning and Definition
Lymphocytes are a type of white blood cells present in the blood and lymph of our body. They are responsible for adaptive or acquired immunity. They are a type of agranulocytes. Around 20-25% of white blood cells are B and T lymphocytes. 99% of the cells of lymph are lymphocytes.
There are three types of lymphocytes, i.e. B-lymphocytes, T-lymphocytes and Natural killer cells (NK cells). They vary in their function and structure. There are surface proteins present, which differentiate the different subtypes of lymphocytes. They are known as the cluster of differentiation or CD markers.
Lymphocytes are concentrated in the lymphoid organs, e.g. spleen, lymph nodes, tonsils, etc. and initiate the immune response against the foreign pathogen.
All the lymphocytes are produced from the stem cell in the bone marrow and later mature and differentiate in the specific organs. B- lymphocytes mature in the bone marrow, whereas T-lymphocytes mature in the thymus. B and T lymphocytes later differentiate into effector and memory cells on exposure to antigens.
Lymphocytes are responsible for both humoral (antibody-mediated) and the cell-mediated immune (CMI) response by B and T lymphocytes, respectively.
It is in the bone marrow that lymphocytes develop. It matures and leaves to enters into the bloodstream. In the blood and different parts of lymphatic system, mature lymphocytes can be found. When these lymphocytes move to the thymus gland, it turns into T cells and when it moves to the lymph structures, they turn into B cells.
In simpler words,
On the basis of structure and function, lymphocytes are divided into three main types:
- B Cells
- T Cells
- Natural Killer (NK) Cells
Let’s learn more in detail about each of these lymphocytes.
B cells get their name from the site of maturation in the birds, where they were first discovered, i.e. bursa of Fabricius. In humans and some other mammals, the main site of B lymphocytes maturation is the bone marrow.
The mature B cell synthesizes and expresses the specific antibodies produced in response to the antigen. It binds to the specific antigen by the membrane-bound immunoglobulin or antibody, which is also known as BCR or B-cell receptor.
The activated B cells further differentiate into plasma cells or effector cells. They lose the surface antibody and start producing the specific antibodies in a large amount to fight the infection.
B lymphocytes produce antibodies, hence they are known to trigger the humoral immune response.
Memory B cells are formed after primary infection and they remain in the blood for decades. They circulate in the blood, identify and act against previously infected antigens.
T cells also get their name from the site of maturation, i.e. thymus. T cells also have surface receptors to recognize antigens but they do not directly bind to the antigens like surface antibody receptors on B cells.
T cell receptors recognize antigens bound to a special kind of cell membrane protein known as major histocompatibility complex or MHC molecules.
T lymphocytes differentiate into two main subtypes:
- T helper (TH) cells- They generally contain CD4 membrane glycoprotein on their surface and recognise antigens with class II MHC.
The major function of T helper cells is to trigger different types of immune cells to act against the antigens like macrophages, B lymphocytes and cytotoxic T cells. The effector T cells secrete different types of cytokines, which directs the immune response by other cell types.
- T cytotoxic (TC) cells- They generally contain CD8 membrane glycoprotein on their surface and recognise antigens with class I MHC.
After activation, they proliferate and differentiate into cytotoxic T lymphocytes. It eliminates virus-infected cells, tumour or cancerous cells and also foreign grafts, etc.
Another type of T cells known as the regulatory T cell, functions as the regulators of immune responses.
The memory T cells are antigen-specific T cells which have a longer life span. They play a key role in rapid immune response on the re-exposure of an infectious agent. They are responsible for the secondary response.
Natural Killer Cells
Natural killer or NK cells are a part of innate immunity and they do not have antigen-specific receptors present on the surface. They play a major role in eliminating tumour cells and infected cells. They distinguish the normal cells from the infected or cancerous cells by MHC class I surface molecules, which is absent in most of the abnormal cells.
NK cells are also activated by cytokines known as interferons. Activated natural killer cells release cytotoxic granules, which kill infected cells.
Function of Lymphocytes in Blood
Lymphocytes play a major role in adaptive immunity. The B cells are responsible for humoral immunity and T cells are required for the cell-mediated immunity. The B cells secrete antibodies, which are passed by blood, consequently they can function over a long distance, while T cells can move to the target tissues and they locally act.
The acquired immune response is specific to the pathogen. Once acquired, it is stored in the memory cell, so that when the infection is repeated, it triggers a highly amplified secondary response. This is because when memory cells encounter the antigen again, it immediately identifies it and responds.
Lymphocytes can bind antigens and aid in removing them from the body through the receptor molecules found on their surfaces. Every lymphocyte has receptors which associate with a particular antigen. This potential to be responsive to virtually any type of antigen is derived from the great diversity in the population of lymphocytes contained in the body. Each of it has a receptor which can identify unique antigens.
The lymphocyte cells aid the immune system of the body and facilitate the opposition against foreign particles and cancerous cells. They are mainly involved in serving as a part of the immune system and accurately responding to foreign invaders entering the body and eliminating them. It produces particular antibodies which aids in protecting against the infectious diseases. T-lymphocytes aid in cell-mediated response while B-lymphocytes aid in humoral immunity.
The normal lymphocyte count is 1000 to 4800 per µl. A decrease or increase in the lymphocyte count has a clinical significance. Lymphocytosis is the increase in lymphocyte count which is an indication of viral infection and also can be as a result of leukaemia. Lymphocytopenia is the decrease in the lymphocyte count from the standard or normal range. It is linked with immune deficiency disorders, malnutrition and some other inherited disorders, etc.
Significance of Lymphocytes
Lymphocytes play a significant role in the immune system. Lymphocytes are a type of white blood cells which are produced in the bone marrow, they circulate through the lymphatic system to fight against diseases and infections. It is inclusive of T cells and B cells that produce antibodies and target foreign invaders. Precisely, the T cells regulate the response of immune system of the body and directly attack and destroys the tumor and infected cells.
On the other hand, the B cells produce antibodies; they are proteins which target bacteria, viruses and other foreign substances.
This was all about Lymphocytes. For more such topics related to NEET, visit BYJU’S. Check NEET Biology Flashcards for the revision of important concepts.
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Frequently Asked Questions
What are lymphocytes?
It is a type of leukocyte present in the immune system of most vertebrates. These cells are concentrated in the lymphoid organs, e.g. spleen, tonsils, lymph nodes, etc.
What are the other names of lymphocytes?
Lymphocytes are a type of white blood cell (Leukocytes).
Lymphocytes are involved in adaptive immunity. The B lymphocytes are responsible for humoral immunity and T lymphocytes are required for cell-mediated immunity. The natural killer cell functions in cytotoxic innate and cell-mediated immunity.
What does it mean when your lymphocytes are high?
The condition of having a high lymphocyte count is known as lymphocytosis. It primarily indicates that our body is fighting with an infection. On a deeper level, it can indicate some serious conditions like leukaemia.
What does it mean when lymphocytes are low?
The condition of having a low lymphocyte count is called lymphocytopenia. It makes it difficult for the body to fight infections.
What is a normal lymphocyte count?
Normal lymphocyte count for adults is between 1100 to 4800 cells per microlitre of blood. For children the normal lymphocyte count ranges between 3000 to 9500 cells per microlitre of blood.
Which blood cells produce antibodies?
Antibodies are produced by special white blood cells referred to as B lymphocytes or B cells. B lymphocytes produce antibodies. The effector B cells can start to secrete antibodies when they are smaller lymphocytes contrary to their end stage of maturation pathway is a large plasma cell that secretes antibodies continuously at the rate of close to 2000 molecules each second.
Are lymphocytes formed in the bone marrow?
Lymphocytes are mature cells which fight against infections. They develop from the lymphoblasts which are a kind of blood stem cell seen in the bone marrow.
Where do neutrophils and lymphocytes originate from?
The cellular constituents of blood, including the white blood cells of the immune system ultimately are derived from the same precursor cells which are the hematopoietic stem cells in the bone marrow.