Views: 3
Lymphoid Organs ,Tissues and Immune Cells
Immune system consist of interconnecting network of organs and tissues . The dominant cells in these organs and tissues are lymphocytes and therefore these organs and tissues are known as lymphoid .
The immune system consist of a number of lymphoid organs including
- Thymus glands
- Lymph nodes
- Spleen
- Tonsils
- Aggregates of lymphoid tissues in noon lymphoid organs e.g. payers patches in the gut
- Bone marrow tissues in the long bones
The thymus and the bone marrow are referred to as the central or primary lymphoid organs or system while the rest are referred to as peripheral or secondary lymphoid organs . These is because development and function of the peripheral lymphoid organs depends absolutely on the cells made by the thymus and the bone marrow
The Primary (Central ) Lymphoid Organs
- Thymus Glands
Thymus gland is a lobed organ located in the thoracic cavity just above the heart. It consists of many lobes each of which contains a cortex and medullar.
Thymus gland begins to function in the embryonic stage but its at its most active stage at time near or shortly after birth.
It however decreases in size after weaning period and will soon stop functioning . Thymus gland is important in the development of immune response in an individual . removal of thymus is called thymectomy . absence of thymus gland in new born leads to premature death due to lack of lymphocytes in the blood and tissue fluids (lymphopenia ) the animal may also lack ability to recognize and react to tissue transplant (does not reject allografts ) but these however is not the case in older animals these may be due to the reason that the function of the thymus glands are carried out earlier in the animals life and hence the thymus becomes less important so long as the remaining parts of the immune system are functional .
The thymus glands functions by accepting the stem cells from the bone marrow at the time of birth and ensures completion of the development . They increase in sizes and develop to mature T – lymphocytes (thymocytes ) . These thymocytes package themselves in the medullar and cortex of the thymus glands . Some of the thymocytes mature into different classes of mature T-cells e.g. helper T-cells , suppressor T-cells and killer T –cells . Others leave the thymus gland through the blood stream and migrates into the lymph nodes , spleen , tonsils , payers patches etc. where they form mature T-cells
T-cells are involve both in the cell mediated and humoral immune system . during the process of maturation , the T-cells becomes immunocompetent i.e. they gain the ability to respond to specific antigens , they also gain the ability to distinguish between self and non self antigens in the body . each T-cells produces small proteins molecules called lymphokynes (or cytokynes or interleukins ). Lymphokynes serve to make T- cells keep on multiplying , promote inflammation and stimulates B-cells to start making antibodies
- Bursa of Fabricious
These is another primary lymphoid organ found in birds in addition to thymus. It is situated near the cloaca . Stem cells enter the bursa of fabricous where they differentiate into B-cells that are capable of producing antibodies.Removal of bursa of fabricous is known a bursectomy. Bursectomised a birds lack B-cells and therefore cannot produce antibodies but they are able to reject graft .
Mammals including man have no bursa of fabricous yet they have B –cells These means that in mammals the bone marrow serves an equivalent role as the bursa of fabricous
- The Bone Marrow
Erythrocytes , leucocyte and platelets are produced in the bone marrow by a process called hemoporesis . The stem cells that give rise to these blood cells are called hemopoetic stem cells .
Precursors of the lymphocytes originates in the bone marrow but they latter migrate in the blood stream to the thymus where they differentiate into T- lymphocytes.
When these stem cells migrate to the spleen or lymphnodes or remain in the bone marrow , they differentiate into B-lymphocytes . The formation of lymphoid outside the bone marrow is known as lymphoporesis .
The blood cells formation depends on the presence of hemopoetic stem cells in the bone marrow (precursor cells)
Earlier in the 19th and 20th century, blood cells were put into two categories according to their presumed origin i.e.
- Erythrocytes , granular leukocytes and platelets developing from the bone marrow were called myeloid and their formation was referred to as myelopoesis .
- Lymphocytes and monocytes were believed to originate in the lymph nodes , spleen and thymus . They were hence referred to as lymphoid elements of blood and therefore there formation was called lymphopoeisis.
But however recent studies have shown that lymphopoetic stem cells similarly originates from the bone marrow
Secondary Lymphoid Organs
- Lymph nodes
The net work of the lymph vessels in the body forms the lymphatic system . At some points in the lymphatic system ,are some swellings referred to as lymph nodes . Lymph nodes are located in some strategic places in the body such as in the armpits ,neck regions and groins etc . Each lymph node receives a flow of lymph through its afferent lymph vessels.
Each lymph node is a spherical or bean shaped and it is surrounded by a tough membrane or capsule . Beneath these capsule is a network of phagocyte lined mesh work called subcapsular sinus . These meshwork region filters any microorganisms present and phagocytes or engulfs them . phagocytes present in these region are macrophages .
The lymph nodes have a densely packed lymphocytes and macrophages organized into spherical nests or cells called follicles. When stimulated by an antigen , these follicles becomes enlarged and the cells inside multiply rapidly particularly at the center (germinal center )
An active follicle produces lymphocytes and plasma cells which in turn produces antibodies . The antibodies are produced as a result of presence or introduction of any antigen . These explains why armpits lymph nodes due to a wound on a finger .the wound allows entrance of antigens into the body which find their way into the lymph node were they are trapped . In the lymph nodes the follicles becomes very active by enlarging and producing many lymphocytes and macrophages . The lymphocytes differentiates into plasma cells which then make antibodies which shall fight against the antigens introduced . The macrophages engulf the bacteria at faster rate .all these activities makes the lymph nodes to swell and become painful
- Spleen
The spleen has two major functions ;
- It destroys old red blood cells
- It’s a major site for immune response
the spleen function just like the lymph node but the main difference is that the fluid it deals with is blood and not lymph .The spleen have two major parts (I) the red pulp and (ii) the white pulp
The red pulp deals with destruction of red blood cells while the white pulp is where the immune response are generated . The lymphocytes are concentrated in the white pulp which surrounds the central artery called periarterial lymph node .
The T- lymphocytes are found in the central parts of the pulp while B-lymphocytes are found in the germinal centers of the lymph follicles . The white pulp is separated from the red pulp by a marginal zone . The lymphocytes enter and leave the pulp via the capillaries of the central arteries in he marginal zone . Some mature plasma cells pass from the marginal zone to the red pulp .
The marginal zone is where the blood lymphocytes ,antigens and tissue fluids are deposited , there also intense phagocytic activities carried out by macrophages
Other lymphoid tissues
- Gut associated lymphoid tissues (GALT)g. payers patches and appendix .
These tissues contain both T and B independent areas which contain T and B lymphocytes
- Mucosa Associated Lymphocytes Tissues (MALT)
They are found in the gut, urinary tract and respiratory tract . they have both T and B lymphocytes