Organs and Tissues of the Immune System

 Organs and Tissues of the Immune System

• Based on function, the organs and tissues of the immune system can be divided into primary or secondary lymphoid organs and tissues.

• The primary organs and tissues are where immature lymphocytes mature and differentiate into antigen sensitive B and T cells.
• The THYMUS is the primary lymphoid organ for T cells, and the BONE MARROW is the primary lymphoid tissue for B cells.
• The secondary organs and tissues serve as areas where lymphocytes may encounter and bind antigen,
• which triggers their proliferation and differentiation into fully active,antigen-specific effector cells.

• The SPLEEN is a secondary lymphoid organ, and
• LYMPH NODES AND MUCOSAL-ASSOCIATED TISSUES (GALT- gut-associated lymphoid tissue, and SALT-skin-associated lymphoid tissue) are secondary lymphoid tissues.

Primary Lymphoid Organs and Tissues

• THYMUS - Primary Lymphoid Organs :
• The thymus is a highly organized lymphoid organ located above the heart.
• Precursor cells from the bone marrow migrate into the outer cortex of the thymus, where they proliferate.
• As they mature, about 98% die.This is due to a process known as thymic selection in which T cells that could recognize and respond to the host (self) are destroyed.
• The remaining 2% move into the medulla of the thymus where they mature and subsequently enter the bloodstream.

• These T cells recognize and respond to the myriad of foreign nonhost substances, collectively referred to as "nonself:'
•   In mammals, the bone marrow is the site of B-cell maturation.
• Like thymic selection during T-cell maturation, a selection process within the bone marrow eliminates nonfunctioning B cells and those that recognize and respond against self.
• Remaining B cells mature in the bone marrow and subsequently enter the bloodstream.

Secondary Lymphoid Organs and Tissues

Secondary lymphoid organ: (SPLEEN & LYMPH NODES)

SPLEEN (Secondary lymphoid organ)

• The spleen is the most highly organized secondary lymphoid organ.
• It is a large organ located in the abdominal cavity that functions to filter the blood and trap blood-borne particles to be assessed for foreignness by phagocytes.
• Macrophages and dendritic cells are present in abundance, and once trapped by splenic macrophages or dendritic cells, a pathogen is phagocytosed, killed, and digested.
• The resulting antigens are presented to lymphocytes, activating a specific immune response.

LYMPH NODES (Secondary lymphoid organ)

• Lymph nodes lie at the junctions of lymphatic vessels, where

macrophages and dendritic cells trap particles that enter the lymphatic system.

• If a particle is found to be foreign, it is then phagocytosed and degraded,
• and the resulting antigens are presented to lymphocytes.

Secondary Lymphoid Tissues : (SALT & MALT)

• Lymphoid tissues are found throughout the body as highly organized or loosely associated cellular complexes.
• Some lymphoid cells are closely associated with specific tissues such as skin (skin-associated lymphoid tissue, or SALT) and

  mucous membranes (mucosal-associated lymphoid tissue, or MALT).

• SALT and MALT are good examples of highly organized lymphoid tissues that feature macrophages surrounded by specific areas of B and T lymphocytes and sometimes dendritic cells.
• Loosely associated lymphoid tissue is best represented by the bronchial- associated 

• lymphoid tissue (BALT), because it lacks cellular partitioning. The primary role of theselymphoid tissues is to efficiently organize leukocytes to increase interaction between the innate and the adaptive arms of the immune response.
• Thus, the lymphoid tissues serve as the interface between the innateresistance mechanisms and adaptive immunity of a host.