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).

The cells of the immune system have following general characteristics:
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).

Mucosa-Associated Lymphoid Tissues (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.

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