Primary Lymphoid organs

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Primary Lymphoid organs
1. To give an overview on the importance of the primary lymphoid organs on generation of the Immune response.
2. To learn about the classification of lymphoid organs including both primary and secondary lymphoid organs.
3. To understand the components of primary lymphoid organ, which include the bone marrow and Thymus gland.
4. Immunological function of both bone marrow and Thymus gland will be review.

The immune system consists of a series of organs and vessels that connect them. Organs of the immune system have been divided into primary and secondary lymphoid tissue. The primary lymphoid organs in most mammals are consisting of thymus gland and bone marrow.

The term of primary is used because these are organs in which naïve lymphocytes are developed; B cells developed in the bone marrow and T cells in thymus gland. Once lymphocytes are formed they migrate to the secondary lymphoid organs and tissue.
Peripheral or secondary lymphoid tissue includes (lymph nodes, spleen, gastrointestinal and mucosa-associated lymphoid tissue)
Lymphocytes are wholly responsible for the specific immune recognition of pathogens and foreign material, so they initiate adaptive immune response. They represent about 20% of the total white blood cells (leukocyte) present in the adult circulation.

Two main types of lymphocytes:

  1. B-cells. Developed and mature in Bone marrow, they produce antibodies

  2. T cells. Originate in bone marrow and migrate to thymus to complete their development and maturation their.

T-lymphocytes have a number of functions including 1) helping B cells to make antibody , 2) recognizing and destroying cells infected with virus, 3) activating phagocytes to destroy the pathogens they have taken up and 4) controlling the level and quality of the immune response.

After Maturation both T and B-lymphocytes migrate from primary lymphoid organ (Thymus & bone marrow) to secondary lymphoid organs though blood circulation.

Primary lymphoid organs.
The primary lymphoid organ is sites where lymphocytes differentiate from hematopoietic stem cells (HSC) are proliferating and mature into effectors cells. From birth to old age, these functions are carried out only in the bone marrow and thymus.
A. Bone marrow (BM).
All lymphocytes arise from HSC in BM, the B cells continue their development, maturation and their function in the bone marrow, where T-cell migrate to Thymus gland to continue their maturation and their function.

In birds, B cells differentiate into the bursa of Fabricious, hence the term of B cells.

The early development of HSC is located in yolk sac (in embryo), then shift to liver.

liver remain the major site of fetal hemopoiesis until shortly before birth when HSC travel by blood to the spleen and then to bone marrow (BM). The bone marrow remains the primary site of hematopoisis in adult until death.

Location of bone marrow.

At birth, the site of all bone marrow is found mainly in the flat bones, such as the hip bone, skull, ribs, sternum and shoulder blades, and in ends of the long bones such as the femur and humerus.

Structure of bone marrow.
There are two types of bone marrow:

  1. red marrow (consisting mainly of with proliferating and differentiating blood cells in connective tissue matrices bordered by venous sinuses, Red blood cells, platelets and most white blood cells (Plasma cells and macrophages) arise in red marrow)
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