Biology
Lymphocytes:
Q.1. What are lymphocytes?
Ans: Lymphocyte is a type of white blood cell that is involved in the immune response; unlike phagocytes, they become active only in the presence of a particular antigen that 'matches' their specific receptors or antibodies.
Q.2. What is B-lymphocyte?
Ans: A type of lymphocyte that gives rise to plasma cells and secret antibodies.
Q.3. What is T-lymphocyte?
Ans: A lymphocyte that does not secret antibodies; T helper lymphocytes stimulate the immune system to respond during an infection, and killer T lymphocytes destroy human cells that are infected with pathogens such as bacteria and viruses.
Lymphocytes are the second type of white blood cell. They play an important role in the immune response.
Lymphocytes are smaller than phagocytes. They have a large nucleus that fills most of the cell. There are two types of lymphocytes, both of which are produced before birth in the bone marrow.
B-lymphocytes (B cells) remain in the bone marrow until they are mature and then spread throughout the body, concentrating in lymph nodes and the spleen.
T-lymphocytes (T cells) leave the bone marrow and collect in the thymus where they mature. The thymus is a gland that lies in the chest just beneath the sternum. It doubles in size between birth and puberty, but after puberty, it shrinks.
Only mature lymphocytes can carry out immune responses. During the maturation process, many different types of B and T- lymphocytes develop, perhaps many millions. Each type is specialized to respond to one antigen, giving the immune system as a whole the ability to respond to almost any type of pathogen that enters the body.
When mature, all these B and T cells circulate between the blood and the lymph. This ensures that they are distributed throughout the body so that they come into contact with any pathogens and with each other.
Immune responses depend on B and T cells interacting with each other to give an effective defense. We will look in detail at the roles of B and T cells and how they interact in the following section. Briefly, however, some T cells coordinate the immune response, stimulating B cells to divide and then secrete antibodies into the blood; these antibodies recognize the antigens on the pathogens and help to destroy the pathogens. Other T cells seek out and kill any of the body's own cells that are infected with pathogens. To do this they must make direct contact with infected cells.
B-lymphocytes:
As each B cell matures, it gains the ability to make just one type of antibody molecule. Many different types of B cell development in each of us, perhaps as many as 10 million. While B cells are maturing, the genes that code for antibodies are changed in a variety of ways to code for different antibodies. Each cell then divides to give a small number of cells that are able to make the same type of antibody. Each small group of identical cells is called a clone. At this stage, the antibody molecules do not leave the B cell but remain in the cell surface membrane. Here, part of each antibody forms a glycoprotein receptor, which can combine specifically with one type of antigen if that antigen enters the body, there will be some mature B cells with cell surface receptors that will recognize it.
When the antigen enters the body for the first time the small numbers of B cells with receptors complementary to the antigen are stimulated to divide by mitosis. This stage is known as clonal selection. The small clone of cells divides repeatedly by mitosis in the clonal expansion stage so that huge numbers of identical B cells are produced over a few weeks.
Some of these activated B cells become plasma cells that produce antibody molecules very quickly - up to several thousand a seconds. Plasma cells secrete antibodies into the blood, lymph, or onto the linings of the lungs and the gut. These plasma cells do not live long, after several weeks their numbers decrease. The antibody molecules they have secreted stay in the blood for longer, however, until they too eventually decrease in concentration.
T- lymphocytes:
Mature T cells have specific cell surface receptors called T cell receptors. T cell receptors have a structure similar to that of antibodies, and they are each specific to one antigen. T cells are activated when they encounter this antigen in another cell of the host. Sometimes this cell is a macrophage that has engulfed a pathogen and cuts it up to expose the pathogen's surface molecules, or it may be a body cell that has been invaded by a pathogen and is similarly displaying the antigen on its cell surface membrane as a kind of 'help' signal. The display of antigens on the surface of cells in this way is known as antigen presentation. Those T cells that have receptors complementary to the antigen respond by dividing by mitosis to increase the number of cells. T cells go through the same stages of clonal selection and clonal expansion as clones of B cells.
helper T cells
killer T cells