ORGANS OF THE IMMUNE SYSTEM

 

Organs of the Immune System:

Primary Lymphoid Tissues:

Bone Marrow------------------ B cells

Thymus------------------------- T cells

Secondary Lymphoid Tissues:

PRIMARY LYMPHOID ORGANS:

Organization is simple as they are sites of growth and maturation and not sites where cells and Ag need to mix.

THYMUS:

T cell progenitor cells enter the Thymus from Bone Marrow to mature and to be "educated". The thymus is a flat, bilobed organ situated over the heart. Lobes are covered by a CAPSULE and are divided into LOBULES by connective tissue strands called TRABECULAE . The lobules are divided into 2 regions;

Cortex:

Medulla:

Both the cortex and medulla are composed of a mesh of cells forming the STROMA or framework of the tissue. These cells include epithelial cells, Interdigitating cells, and macrophage. These cells make contact with thymocytes to stimulate growth, carry out +ve and -ve selection, and remove dead cells (90-95% of cells die). Chapters 9 and 10 will deal with the details of thymic development.

BONE MARROW

The Bursa of Fabricius is the site of B cell development in birds. The bone marrow and possibly other sites (Peyer's patches in sheep and cattle) play this role in mammals, but the lack of information on the organ and how it works limits our knowledge of B cell development. Details of B cell development and selection will be discussed later (chapter 11).

SECONDARY LYMPHOID ORGANS:

Primary follicle-- network of follicular dendritic cells and small resting B cells not yet stimulated by Ag.

Secondary follicle-- Ag stimulated ring of B cells around a Germinal Center of proliferating B cells, B memory cells, and plasma cells, mixed with macrophage and follicular dendritic cells.

LYMPH NODE

Architecture provides microenvironment for lymphocytes to effectively encounter and respond to Ag.

SPLEEN

MUCOSAL-ASSOCIATED LYMPHOID TISSUE (MALT)

Tonsils:

Peyer's Patches: