Lymphocytes | Guardians of Immunity

Lymphocytes are the White Blood Cells also called as Leukocytes. Which comprises of Natural Killer cells B and T cells. These are very important for maintaining the immune system because these are the cells which fight against,  identify and kill the infectious micro-organisms and other foreign substances. Bone marrow constantly produces cells that will become lymphocytes. Some will enter in bloodstream, but most will move through  lymphatic system. The lymphatic system is the group of tissues and organs, like the spleen, tonsils, and lymph nodes, that protect our body from infection. About 25 percent of the new lymphocytes remain in the bone marrow and become B cells. The other 75 percent travel to  thymus and become T cells.

The two main types of lymphocytes are known as T and B cells. These two types of lymphoid cells are developed and differentiated in the primary lymphoid organs. For example, T cells are developed in the thymus, where as the B lymphocytes are differentiated in the adult bone marrow and fetal liver. In birds, B cells are differentiated in the bursa of Fabricius. Further more, in the primary lymphoid organs T and B cells precursors acquire the ability for recognizing antigens through the development of specific surface receptors. NK- cells do not express antigen receptors on their cell membranes. They are capable to lyse certain tumor cell lines in vitro without being sensitized. NK cells are large granular lymphocytes (LGLs) Lymphocytes are produced in the primary lymphoid organs (thymus and adult bone marrow) at a high rate 109 per day. Some of these cells migrate to the blood circulation via the secondary lymphoid tissues (spleen, lymph nodes, tonsils, and mucosa-associated lymphoid tissue). The average human adult has about 1012 lymphoid cell, and approximately 2% of the body weight is a lymphoid tissues. Lymphoid cells represent about 20% of the total leukocytes population in the adult circulation. Many mature lymphoid cells are long-lived, and may persist as a memory cells for several years, or even for the live time of the individual. 

Availability of Lymphocytes in the Body 

• Lymphocytes in the Blood Stream  
• Lymphocytes Outside the Bloodstream, in the Lymph vessels   
• Lymphocytes in the Lymph Nodes

What lymphocytes do in the body ?

There are actually many differences between B-cells and T-cells, even though they are both lymphocytes. B-cells and T-cells are associated with different parts of the immune system. One part of the immune system—the more B-cell dominant part—is focused on making antibodies that can bind to foreign invaders and lead to their destruction. The other part of the immune system—the more T-cell dominant part—is focused on recognizing the invaders and then directly killing them, through a very specific recognition sequence that leads to cell-to-cell battle. These two different turfs or part are described by specific terms. The artillery, or the antibody-producing side, is known as humoral immunity. The infantry, or the cell-to-cell battle side, is known as cell-mediated immunity.

B-cells are the cells that come to mind when thinking about antibodies, or humoral immunity, and T-cells are the cells that come to mind when thinking about cell-to-cell combat, cytotoxicity, or so-called cell-mediated immunity. In reality, there is often cooperation between B-cells and T-cells, just as there is coordination between those who fire the mortars and the infantry.

B-cells mature in the bone marrow and move to the lymph nodes. B-cells become plasma cells or memory cells when foreign antigens activate them; most B-cells become antibody-producing plasma cells; only some remain as memory cells. Memory B-cells help ensure that if the enemy is encountered again in the future, the mortars are prepared. Plasma cells can be found in lymph nodes and elsewhere in the body, where they work to produce large volumes of antibodies. Once antibodies are released into the blood and lymph, these antibody molecules bind to the target antigen to begin the process of neutralizing or destroying the foreign agent.

T-cells mature in the thymus and differentiate into different types. There are several types of T cells, including the following:

Cytotoxic T cells find and directly attack foreigners such as bacteria, viruses, and cancer cells.

Helper T cells recruit other immune cells and organize an immune response.

Regulatory T cells are thought to suppress the immune system so that it doesn't overreact (as it does in autoimmune diseases), however central aspects of the biology of these cells remain shrouded in mystery and continue to be hotly debated.

Natural killer T (NKT) cells are not the same thing as natural killer cells, but they do have similarities. NKT cells are cytotoxic T cells that need to be pre-activated and differentiate to do their work. Natural killer (NK) cells and NKT cells are subsets of lymphocytes that share common ground. Both can rapidly respond to the presence of tumor cells and participate in anti-tumor immune responses.

Memory T cells remember markers on the surface of bacteria, viruses, or cancer cells that they have seen before.

Difference between B and T cell

S.No	B Cell	T Cell
1.	Responsible for humoral immunity	Responsible for cell mediated immunity
2.	Life span is short	Life span is long
3.	Differentiate inside bone marrow	Differentiate inside thymus gland
4.	Surface antibodies present	Absence of surface antibodies
5.	Transformed into plasma cells by antigens	Transformed in small lymphocytes by antigens
6.	Secrete antibodies	Secrete Lymphokines
7.	Sub population are memory cells and plasma cells	Sub population are cytotoxic T,  Helper cells and Suppressor cells
8.	B-Cells or B-lymphocytes produce antibodies	Stimulates phagocytes and B cells in to activity