The body has a system of cells—the
immune system—has the ability to neutralize or inactivate foreign molecules
(such as soluble molecules as well as molecules present in viruses, bacteria,
and parasites) and to destroy microorganisms or other cells (such as
virus-infected cells, cells of transplanted organs, and cancer cells).
1. They are distributed throughout the body in
the blood, lymph, and epithelial and connective tissues;
2. They are arranged in small spherical nodules
called lymphoid nodules found in connective tissues and inside several
organs and
3. They are organized as differently sized
organs called lymphoid organs— like the lymph nodes, the spleen, the
thymus, and the bone marrow.
a. Spleen and thymus are called primary or
central lymphoid organs.
b. Lymph nodes, spleen and other lymphoid
tissues line tonsil are called secondary or peripheral lymphoid organs.
Lymphoid
Tissue
Lymphoid
tissue is a type of connective tissue characterized by a rich supply of
lymphocytes. Lymphoid tissue
exists free within the regular connective
tissue or is surrounded by capsules, forming the lymphoid organs.
stains dark blue in hematoxylin and
eosin-stained sections owing to sparse cytoplasm in the constituent cells i.e.
lymphocytes.
are basically made up of free cells and
have a rich network of reticular fibrils
(made principally of type III collagen) that supports the cells.
Lymphoid Nodules or Lymphoid
Follicles
The Nodular lymphoid
tissue has groups of lymphocytes that are arranged as spheres, called lymphoid
nodules or lymphoid follicles, or lymphatic nodules (See the figure-3 below showing a section from
lymph node) that primarily contain B lymphocytes. When lymphoid
nodules become activated as a result of the arrival of antigen-carrying APCs
and recognition of the antigens by B lymphocytes, these lymphocytes proliferate
in the central portion of the nodule, which then stains lighter and is called a
germinative center. After completion of the immune response, the
germinative center may disappear. The germinative centers contain a special
cell, the follicular dendritic cell (distinct from the epithelial
dendritic APCs), that has many processes that bind antigen on their surfaces,
to be presented to B lymphocytes.
Lymphoid
nodules vary widely in size, typically measuring a few hundred micrometers to 1
mm in diameter. They are found free in connective tissues anywhere in the body
(diffuse lymphoid tissue) or within lymphoid organs (lymph nodes, spleen,
tonsils, but not in the thymus).
Free
lymphoid nodules are commonly present in the lamina propria of several mucosal
linings, where, together with free lymphocytes, they constitute the mucosa-associated
lymphoid tissue (MALT).
The
digestive, respiratory, and genitourinary tracts are common sites of microbial
invasion because their lumens are open to the external environment. To protect
from the organism, the mucosa and submucosa of these tracts contain a large
amount of diffuse collections of lymphocytes, IgA-secreting plasma cells, APCs,
and lymphoid nodules. Most of the lymphocytes are B cells; among T cells, CD4+
helper cells predominate.
In some
places, these aggregates form conspicuous structures such as the tonsils and
the Peyer's patches in the ileum. Similar aggregates are found in the appendix.
Bronchial Associated Lymphoid Tissues (BALT) and Gut Associated
Lymphoid Tissues (GALT) are only the specialized name for MALT associated with respiratory and GI tract respectively.
REFERENCES:
Following text books are used while preparing this post (readers are strongly recommended to go through them for more details):
Junqueira's
Basic Histology: Text and Atlas, 13th Edition
Wheater's
Functional Histology: A Text and Color Atlas, 5th Edition