Lymphoid tissue and lymphoid organs

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Lymphoid tissue and lymphoid organs

The lymphatic system is specialized form of connective tissue that consists of group of cells, tissues and organs.

  • It monitors the body surface and internal fluid compartments and reacts to the presence of potentially harmful antigens included in this system: thymus, spleen, lymph nodes, lymphatic nodules and diffuse lymphatic tissue.

  • The lymphatic tissue & organs are collectively referred as the immune system including the lymphoid cells present throughout the circulation.

What is lymph = lymph is a transudate from the blood containing crystalloid materials containing same protein of plasma.

Suspended in the lymph are the chief cellular components of the lymphatic tissues, lymphocytes they circulates within the lymphatic vessels, scattered along the lymph vessels are numerous small bean-shaped structure called lymph nodes.

  • Lymph nodes act as fibers removing bacteria and other particulate.

  • Lymph node adds lymphocytes to the lymph.


The bone marrow is the predominant source of stem cells (hemocytoblasts) from which lymphoid cells are derived (And persistence to adult life).

In late fetal life many stem cells migrate from the bone marrow to the primary lymphoid organs.

In the primary lymphoid organs. These immigrant stem cells proliferate & differentiate into immunocompetent cells. These cell are processed

i.e. become competent to recognize and destroy invading microbes and have ability to distinguish between self and nonself molecules.

After processing lymphocytes (both T & B) transported by the cardiovascular system to the secondary lymphoid organ.

The primary or central lymphoid organs


2- Gut associated lymphoid tissue e.g. (Appendix & pyre’s

patches) (Correspond to bursa of fabricius in bird).
3- The liver or bone marrow it self depending on the species

or the stage of development or both.

T Lymphocytes mature in the thymus and so-called thymus

derived cells (T lymphocytes), which are involved in cell-

mediated (cellular immunity).

B lymphocytes (bursa- derived cells) mature in the bone marrow or gut associated lymphoid tissue are involved in humoral immunity and the production of proteins antibodies.

These T & B lymphocytes are called virgin cells (before exposure to antigen).

Secondary lymphatic organ (peripheral)

These include lymph nodes, lymphatic nodules, spleen & tonsils.

In these organs T & B lymphocytes undergo antigen dependent proliferation and differentiation into effectors lymphocytes and memory cells.

Immunological tolerance = when immune system can recognize self antigen but doesn’t attempt to attack them.

Clinical note

Autoimmune system: involve malfunction of immune system

i.e. loss of immunological tolerance, e.g. Grave’s disease

thyrotoxicosis antibodies against receptors of TSH in thyroid


Types of immunity

In cell mediated immunity activated T lymphocytes (effectors T

cells) attaching the antigen either directly or by releasing


Many types of lymphokines e.g.

  1. Lymphotoxin = kill the antigen or lysis of antigen.

  2. Blastogenic factor induce proliferation of non sensitized lymphocytes.

  3. Chemotaxis = attract macrophages and different types of leukocytes which assist lymphocytes in eliminating the antigen.

  4. Interferon (kill cancer cells) see page 222 table.

  • Cellular immunity characterized by localized reaction because the lymphocytes seak out the antigen or risk after the antigen.e.g. tuberculin test and histoincompatible (rejection of transplant).

  • Also it is usually a slow or delayed hypersensitivity reaction.

There are many types of effector T cells:

      • T cell recognize & destroy tumor cell

      • T lymphocyte recognize & fight organism that produce infection.

      • T lymphocytes which regulate immune or allergic response, it secrete interleukin.

      • Helper T lymphocyte = it enhance the immune response recognize the antigen and attract macrophages or B cells to attract antigen.

      • T cells which only attract the antigen & concentrated.

      • T cells which suppress the immune response or inhibit it, T suppressor cells (inhibit attaching self antigen). These can be recognized by unique surface e.g cecule (specific receptors).

      • Memory cells = they don’t responed to initial primary response to an antigen, but they increase the circulating population of pre programmed lymphocytes capable of recognition of a particular antigen and responding to a second exposure.

In humoral immunity:

        • Activated B lymphocytes and plasma cells secrete specific antibodies that circulate, combine with antigen and form inactive complexes.

        • Here not the cells seek out the antigen but instead their secretory products (antibodies) distributed throughout the body fluid and seek out or running after the antigen.

        • The secondary response is usually very rapid and intense and may cause anaphylactic shock such as penicillin, insect venous..

        • Antibodies are proteins found in the plasma and referred to them as immunoglobulins:

- Against bacteria

Or - against bacterial toxin like tetanus

There are 106 – 109 different types of AB in one person.
There are five different classes:

  1. Ig M:

High molecular weight = macroglobulin.

  • It is important in primary response.

  • It is a cytolytic agent (i.e lyses of foreign cells).

  • It plays important role in antigen agglutination and opsonization.

  • Activate complement.

2. Ig G:

  • It is predominant in the serum.

  • Neutralize bacterial toxins and viruses.

  • Important in secondary response.

  • Not activating complement.

  • Cross placenta, so give passive immunity to the fetus and secreted in milk.

3. Ig A:

Two types: Serum Ig A and dimeric form found in seromucous

secretion so called secretory Ig A or s Ig A .

It is stabilize against proteolytic enzyme by combining with

other protein (J protein) protect the body surfaces from

invading of organism. Help esinophils to recognize & kill

4. Ig E:

Play important role in allergy and parasitic infection.

It binds to mast cell and basophile to release histamine

and heparin, help esinophil to recognize & kill parasite.

Its level is very low in the serum.
5. Ig D

  • It is responsible for lymphocyte activation.

  • It is usually found in the surface of B & T lymphocytes to activate B cells to differentiate into plasma cell with Ig M.

AIDS = virus HIV. binds to C D4 molecule of T helper cell


  • It develops from the third branchial arch and migrates caudally.

  • It is a bilobed organ.

  • It is well developed from the time of birth to puberty.

  • It is situated in the midline of the superior mediastinum.

i.e. behind the manubrium sterni.

  • It is usually not seen in cadavers because it undergo rapid autolysis.

It consists of : Stroma and parenchyma

Stroma = Capsule & framework.

  • The thymus surrounded by C.T capsule, septa extend from the capsule to the thymic tissue and divide the thymus into lobules (they are not true lobule). Smaller septa extend into each lobule from main septa.

  • Reticular fibers and cells are absent but instead.

There are epithelioreticular cells which are attached to

each other by desmosomes and form the framework of

the thymus.

  • They are not phagocytic as reticular cells of other lymphoid organ.

  • They are secretory cells produce hormones like substance which regulate T cell development.

  • They are endodermal in origin.


Each lobule is separated into:

  1. A peripheral cortex.

  2. And a central medulla

Because the lobules they are not complete (not true lobule) the medulla become continuous with adjacent lobules. May be in some section the lobule is completely surrounded by cortex and the medulla present exactly in the center and this give confusion with lymphatic nodule with germinal center.
The cortex is intensely basophilic due to the presence of small numerous:

    1. Lymphocytes, lymphocytes have intensely stained nuclei, (thymocytes).

    2. Numerous macrophages which can be displayed with PAS reaction. They are present to phagocyte or destroy lymphocytes which randomly programmed against self antigen.

    3. Epithelial reticular cellular abundant:

Type I, Type II, Type III

The medulla = Stained less intensely than the cortex because, like germinal center of lymphatic nodules, it contains mostly large lymphocytes with paler staining nuclei and with relatively more cytoplasm than in small lymphocyte (fewer).

  • Also they contain epithelioreticular cells with euchromatic nuclei.

  • Few mast cells, macrophage, plasma cells, granular leukocytes may be found in small numbers.

Thymic corpuscles = Hassels corpuscles = unknown function.

  • They are distinction feature of thymic medulla.

  • They are isolated masses of closely packed, concentrically arranged epithelioreticular cells, mainly type VI, these cells stain bright pink in H & E).

  • The center is formed of epithelial cells undergo degeneration + may be calcification

“keratinized center is look like pink stained hyaline
Blood – thymic barrier

The blood vessels pass from the capsule to penetrate and ramify the interlobular C.T. They enter the parenchyma by coursing along the corticomedullary zone.

The arteries extend into the cortex and from a capillary network that extensively anastomose with one another.

  • Subsequently the capillaries drain into the pot capillary venules and veins on the medulla.

  • Antigens that penetrate to medulla are rapidly phagocytoced by macrophages before they can diffuse to the cortex.

  • In the cortex the capillaries are ensheathed by epithelio-reticular cells and a basal lamina is interposed between the epithelioreticular cells and perivascular connective tissue.

All these components form the blood thymic barrier:

    • Capillary endothelium.

    • Endothelial basal lamina

    • Perivascular C.T containing many macrophages.

    • Basal lamina of epithelioreticular cell.

    • Epithelioreticular cell sheath.

Thymic Function

  • During fetal life and childhood, the thymus is the site of programming, differentiation and proliferation of T lymphocytes.

      • These processes are enhanced by hormone like factors, which produce by epithelioreticular cells. These factor enhance the transformation of bone marrow stem cells (pre-T cells into mature T cells, e.g. serum thymus factor, thymopoietin, thymosin, thymulin.

activity of lymphocytes that undergo blastogenesis.
This process don’t involve antigen stimulation in neonate.

So, thymus is important in the immunological maturation of T lymphocytes.

Thymoctomy in neonatal animals showed that the animals are unable to produce immunocompetent T cells and send it to peripheral lymphoid tissue, therefore these animals are susceptible to infection.

At puberty thymus undergo involusion and replaced by adipose tissue. This because the immunocompetent T cells has been established (long life).

Accidental involutions of the thymus occur as a result of sever:

(1) Stress, (2) prolonged disease, ionizing radiation and

(3) deficiencies.

1, 2, & 3 reversed if the cause is removing.

  • Enlargment of thymus after puberty associated with or disease called Mysthenia gravis.

  • Some hormones influence T cell maturation e.g. adrenocorticosteroid – thyroxyne & somatotropin (GH).

  • Congenital failure of the thymus to develop is called Digeorg’s syndrome. The patients can not produce T lymphocytes and usually die at young age from infection.

They also lack parathyroid glands and die from tetany.

The Bursa Equivalent = Bursa of fabricius

BF: It is the avian central lymphoid organ that generates B


The bursectomized chick do not develop the capacity to

produce humoral antibodies.

  • In mammals, an equivalent developmental system exists, possibly in the gut – associated lymphoid tissue.

  • A gammaglobulinemia, in which no humoral immunity develop. Neonate are succeptibal to bacterial infection sever infection such as encephalitis + meningitis.

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