What are lymphoid cells?

Question

Defenition

Answer 1

A lymphocyte is a type of white blood cell involved in the vertebrate immune system. There are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes. The large granular lymphocytes are more commonly known as the natural killer cells (NK cells). The small lymphocytes are the T cells and B cells. Lymphocytes play an important and integral role in the body's defenses.

Types of Lymphocytes
The three major types of lymphocyte are the natural killer (NK) cells, T cells and B cells. NK cells are a part of cell-mediated immunity and act during the innate immune response. They can attack host cells that display a foreign (e.g. viral) peptide on particular cell surface proteins known as MHC class I molecules. Once they determine a cell is infected, the NK cells release cell killing (cytotoxic) granules that will destroy the infected cell. NK cells do not require prior activation in order to perform their cytotoxic effect upon target cells. Like NK cells, the T cells are chiefly responsible for cell-mediated immunity whereas B cells are primarily responsible for humoral immunity (relating to antibodies). T cells are named such because these lymphocytes mature in the thymus; B cells (named for the bursa of Fabricius in which they mature in bird species) are thought to mature in the bone marrow in humans. T and B lymphocytes differ from NK cells in that they are the principal cells involved in the adaptive immune system. These are cell types that retain a memory of a previous infection so that they can respond to the same infectious agent quickly upon reinfection. In the presence of an antigen, B cells can become much more metabolically active and differentiate into plasma cells, which secrete large quantities of antibodies. T cells, after they see an antigen, will also become highly activated and will secrete specific proteins, such as cytokines and cytotoxic granules, depending on their subtype/function.-

Answer 2

If I had to pick one it would be d) Because cell mediated immunity involves the activation of macrophages, natural killer cells (NK), antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen. Historically, the immune system was separated into two branches: humoral immunity, for which the protective function of immunization could be found in the humoral (cell-free bodily fluid or serum) and cellular immunity, for which the protective function of immunization was associated with cells. OK this guy below me is right: Lymphocytes are the cornerstone of the adaptive immune system. They are derived from common lymphoid progenitors. The lymphoid lineage is primarily composed of T-cells and B-cells (types of white blood cells). This is lymphopoiesis. Myelocytes, which include granulocytes, megakaryocytes and macrophages and are derived from common myeloid progenitors, are involved in such diverse roles as innate immunity, adaptive immunity, and blood clotting. This is myelopoiesis. Technically macrophages are myelocytes. Thank you Mr. Crate Rib

Answer 3

They are a part of LEUKOCYTES (the white cells). Leukocytes are the defenders. They split into two categories: Granulocytes and lymphoid cells. Lymphoid cells are then distinguished as either monocytes or lymphocytes. Lymphocytes are #1 when it comes to our immune system. They create to help defend off the bad guys. Their job is to fight off disease. It gets more complex with T-cells and helper cells, etc. Hope this helps!

Answer 4

Leukocytes, or white cells, are responsible for the defense of the organism. In the blood, they are much less numerous than red cells. The density of the leukocytes in the blood is 5000-7000 /mm3. Leukocytes divide in two categories: granulocytes and lymphoid cells or agranulocytes. The term granulocyte is due to the presence of granules in the cytoplasm of these cells. In the different types of granulocytes, the granules are different and help us to distinguish them. In fact, these granules have a different affinity towards neutral, acid or basic stains and give the cytoplasm different colors. So, granulocytes distinguish themselves in neutrophil, eosinophil (or acidophil) and basophil. The lymphoid cells, instead, distinguish themselves in lymphocytes and monocytes.

Each type of leukocyte is present in the blood in different proportions:

neutrophil 50 - 70 %
eosinophil 2 - 4 %
basophil 0,5 - 1 %
lymphocyte 20 - 40 %
monocyte 3 - 8 %

Lymphocytes are cells which, besides being present in the blood, populate the lymphoid tissues and organs too, as well as the lymph circulating in the lymphatic vessel. The lymphoid organs include thymus, bone marrow (in birds bursa), spleen, lymphoid nodules, palatine tonsils, Peyer's patches and lymphoid tissue of respiratory and gastrointestinal tracts.

Most lymphocytes circulating in the blood is in a resting state. They look like little cells with a compact round nucleus which occupies nearly all the cellular volume. As a consequence, the cytoplasm is very reduced. The lymphocytes of the lymphoid tissues and organs can be activated in a different amount following antigenic stimulation. In the blood, lymphocytes are 20-40 % of all leukocytes and are slight larger than red blood cells.

The lymphocytes are the main constituents of the immune system which is a defense against the attack of pathogenic micro-organisms such as viruses, bacteria, fungi and protista. Lymphocytes yield antibodies and arrange them on their membrane. An antibody is a molecule able to bind itself to molecules of a complementary shape called antigens, and recognize them. As for all proteins, even the antibodies are coded by genes. On the basis of a recombination mechanism of some of these genes, every lymphocyte produces antibodies of a specific shape.

Hence, lymphocytes perform an action which is called specific in that each of them recognize the complementary antigen only. Even if every lymphocyte is so selective to recognize only one molecule, the number of circulating lymphocytes is so large that they are able to recognize practically all substances which are in the organism, both its own and foreign. It is a question of recognizing hundreds of millions of different molecules.

The cells of the immune system, chiefly lymphocytes, cooperate amongst themselves to activate, boost or make more precise the immune response. To attain this scope, there exist different types of lymphocytes, with different functions: T and B lymphocytes. When the B cells are activated, they breed quickly (clonal selection) and they become plasmacells which secrete a great deal of antibodies in the blood stream (humoral response). When free antibodies meet micro-organisms with complementary shape (epitopes), they bind to them and form complexes which immobilize the micro-organisms. Later, other cells which are not specific, but which are able to recognize antibodies, phagocyte these complexes.
In their turn, the T cells are divided into three categories: Tc (cytotoxic), Th (helpers), Ts (suppressors). Even the Cytotoxic lymphocytes breed quickly when they are activated. They do not release antibodies in the bloodstream, but they keep the antibodies on their membrane and use them to recognize cells mainly of its own organism infected by virus or tumoral cells. The cytotoxic lymphocytes kill cells by means of the release of perforins, substances which produces lesions in the membrane of the target cell and cause its death by osmotic lysis (cell-mediated response). The helper lymphocytes are needed to activate both B and Tc lymphocytes which, even though they recognize extraneous agents, seldom enter into direct action. Suppressor lymphocytes reduce the intensity of the immune response.

However, the immune system must not attack the cells of it's body as the autoimmune reaction can damage the organism and lead to death. How does the immune system distinguish between self and not self? We have seen that B and Tc lymphocytes which have recognized an antigen, do not enter in action, but they need to be activated by a helper lymphocyte. A few times after the organism's birth, some of the new lymphocytes pass through the thymus where they become T lymphocytes. Here, these cells are compared with all antigens of the organism (autoantigens). It seems that lymphocytes which recognize an antigen, as they are still immature, will die. In this way, as the autoreactive Th lymphocytes are been killed, only the B and Tc lymphocytes which have recognized extraneous antigens can be activated. The system of cellular cytotoxicity mediated by Th cells is evolved as a defense against their own infected, modified or aberrant cells. In fact, B and Tc lymphocytes can activate themselves against bacteria even without the agreement of the helpers.

The B and Tc activated lymphocytes, besides to producing antibodies and killing foreign cells, multiply quickly. During the cellular division, rearrangements often occur in the sequence of the genes which code for the antibody. In this way, the antibody of the new cell takes a slightly different shape in comparison to that of its "mitotic parent". If the new shape matches the antigen better, this cell will be induced to divide more. The next generation of clones is therefore more efficient and, in its turn, can induce more selective varieties. This process and that of clonal selection make the immune response more effective. Finally, the immune system produces memory cells, i.e. deactivated lymphocytes ready to be reactivated on the occasion of further meeting with the same antigen.

Besides the Th and B cells, there is a third population of lymphocytes in the peripheral blood and lymphoid organs which do not have receptors for antigens. These lymphocytes have a non-specific defense function which is not activated by Th lymphocytes. These cells represent the more ancient component of the immune system and they are characterized by their cytotoxic activity. For these reasons, they are named NK, Natural Killer. Apart from killing viruses, bacteria, infected and neoplastic cells, these lymphocytes also regulate the production of other hematic cells such as erythrocytes and granulocytes

Monocytes are the precursors of macrophages. They are larger blood cells, which after attaining maturity in the bone marrow, enter the blood circulation where they stay for 24-36 hours. Then they migrate into the connective tissue, where they become macrophages and move within the tissues. In the presence of an inflammation site, monocytes quickly migrate from the blood vessel and start an intense phagocytory activity. The role of these cells is not solely in phagocytosis because they have also have an intense secretory activity. They produce substances which have defensive functions such as lysozime, interferons and other substances which modulate the functionality of other cells. Macrophages cooperate in the immune defense. They expose molecules of digested bodies on the membrane and present them to more specialized cells, such as B and Th lymphocytes.

Hope this helps.

Answer 5

They are also called lymphocytes.

Read up:
http://en.wikipedia.org/wiki/Lymphocytes

.

Answer 6

lymph cells