11-Arid-975 DVM 3rd Semester

Lymphoid Organs are sites where cells of the immune system originate and develop and where acquired immune responses are initiated.

The lymphoid tissues are divided functionally into:

1) Primary/Central Lymphoid Organs2) Secondary/Peripheral Lymphoid Organs

Primary/Central Lymphoid Organs:

Primary lymphoid organs are the sites where lymphocytes originate or mature.There are two primary lymphoid organs in mammals:

Bone Marrow Thymus

Bone Marrow:

The bone marrow is also the site of B cell development and maturation. Although T-cell precursors originate in the bone marrow, their maturation occurs in the thymus.

Thymus:

Bilobed organ on top of the heart Reaches maximum size during puberty

o 70g in infants,3g in adults 95-99% T-cells die in tymus Consist of cortex & medulla T-cells maturation takes place in thymus

T-Cell Maturatio

T cell precursors leave bone marrow and migrate to the thymus where they mature.

Secondary/Peripheral Lymhoid Organs:

Secondary lymphoid organs are the sites where acquired immune responses are initiated and are scattered throughout the body, but are concentrated in areas that are most likely to be invaded by pathogens (i.e. the gastrointestinal tract, the respiratory tract, & the genitourinary tract).

Secondary lymphoid organs are:

Lumph Nodes Spleen MALT (Mucosal Associated Lymphoid Tissue)

Lymph Node:

Lymph nodes are scattered throughout the body and filter out microbes or damaged tissue.Lymph nodes are responsible for the acquired immune response against antigens.Afferent lymphatics deliver fluid from the tissues to the subcapsular sinus of the lymph node. This fluid contains cells that have encountered pathogens and hence is a source of antigens that stimulate the acquired immune system.

From the subcapsular sinus, the lymph drains into the cortex of the lymph node. The cortex contains B cell-rich areas called follicles.The B cell follicles are in contact with T cell-rich regions,which facilitate the interactions between these two two lymphocytes that are necessary for antibody production. Some follicles contain germinal centers; these are areas where B cells proliferate and differentiate after they have encountered antigen.Activated B cells mature into plasma cells,which secrete large amounts of antibody,and are located in the medulla of the lymph node.Lymphoid fluid leaves the lymph node

through the efferent lymphatics.In addition, activated B and T cells and antibody molecules leave the lymph nodes and enter the peripheral blood.

Spleen:

The spleen is located in the abdominal cavity and acts as a filter

for peripheral blood.The spleen consists of two basic types of tissue.

1. Red pulp, which encompasses most of the splenic tissue,is involved in the degradation of senescent red blood cells.

2. White pulp is scattered throughout the red pulp and this is where the acquired immune response is initiated.The white pulp is organized around central arterioles, which deliver blood (containing lymphocytes and antigen).Like the lymph node, the white pulp of the spleen is organized into T cell-rich regions (periarteriolar lymphoid sheath) and B cell-rich areas (follicles).Antigenic stimulation induces B cell proliferation and the formation of germinal centers.

MALT (Mucosal Associated Lymphoid Tissue):

"MALT" refers to a diffuse collection of lymphoid tissues that line the respiratory, alimentary, and genitourinary tracts.MALT produce the immune responses against pathogens that invade the mucosa that line these tracts.Like the spleen and lymph nodes,the MALT contains B cell follicles and distinct T cell-rich regions.In the intestine we find Peyer's Patches and the tonsils are similar structures in the upper alimentary tract.Although its organization is similar to the spleen and lymph node, the mucosal immune system is different in several ways:

(1) unlike the spleen and lymph node,MALT tissue is not surrounded by a fibrous capsule

(2) IgA is the predominant class of immunoglobulin produced in MALT

(3) It is not filtering Ag delivered by vessels like the blood vessels or lymphatic vessels

BALT (Bronchial Associated Lymphoid Tissue) & GALT (Gut Associated Lymhoid Tissue) are subtypes of MALT.