Lymphoid tissue and lymphoid organs

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Activated B cell

    1. Migrate to the germinal center.

    2. Undergo mitotic divisions.

    3. Give rise immature immunoblast that proliferate and give rise to plasma cells and memory B cells.

Plasma cells migrate to the medullary cords where they synthesize and release specific antibodies into lymph blood

memory B cell circulates to different region and in secondary exposure to the antigen proliferate plasma cells antibodies.
Enlargement of the lymph node is due to proliferation of B lymphocytes and enlargement of the germinal centers.

Gut associated lymphoid tissue

  • These organs (either whole organ or part) represent the bursa equivalent in human and some mammals.

  • These include = tonsils, pyers patches, the appendix, the solitary nodules that scattered throughout the GIT tract numerous in the colon.

In GIT + respiratory tract, the lamina propria contain large number of lymphoid cells and often considered as a lymphoid tissue. Plasma cells here (in this layer) secret secretory IgA to prevent bacteria and viruses to penetrate overlying epithelium. When there is highly organized lymphoid tissue (such as previously mentioned) it is become greatly enlarged.

The tonsils

There are three distinct tonsils. They are forming incomplete ring at the entrance of the or pharynx..
These are palatine (paired)

Lingual (paired)

Pharyngeal (diffuse lymphatic tissue) (single).

  • The palatine tonsil and lingual tonsil are covered with stratified squamous epithelium.

  • Pharyngeal tonsils is covered by pseudostratified columnar ciliated epithelium with goblet cells (nasopharynx)

  • The epithelium of palatine + lingual tonsil underneath skeletal muscle + lingual glands is thrown into folds forming crypts (usually deep), which may bifurcate underneath the epithelium the lymphoid tissue arrange in single layer of lymphatic nodules with germinal centers. The nodules are a symmetrical (unlike lymph node cortex)

  • The covering epithelium is called usually follicular epithelium that contains macrophages, lymphocytes, plasma cells and polymorph nuclear cells.

Peyer’s Patches

The lymphatic nodules are present in solitary form in the lamina

propria and submucous of the small intestines.
Aggregation of these lymphatic nodules in the ileum is called peyer’s patches.
These aggregations (30 – 40 in number) are usually confined to the lamina propria and may extend into the submucosa.
The follicular epithelium associated with the nodules is highly specialized. These are areas where no villus projections occur. While most of the constitute are typical columnar with microvilli. Some are attenuated cells actively traps antigen and present them to the lymphoid organ below.

Palatine Tonsil