Chapter 2: Lymphoid structures

Chapter 2: Lymphoid structures

Anatomy of the immune system:
Primary lymphoid organs are organs where lymphocytes are formed and where they mature.
The primary lymphatic organs include:
·        Bone marrow for B cells
·        Thymus for T cells.

Bone marrow:
Bone marrow consists of two primary structures:
·        Red marrow (major site of blood cell formation)
·        Yellow marrow (stroma and mostly fat)


·        It is derived from the third and fourth pharyngeal (branchial) pouches.
·        It is the primary site of T- cell differentiation and maturation.
·        Immature T-cells move from bone marrow to the cortex of the thymus for positive selection and to mature into CD4 and CD8 T-cells.

Interesting fact: On radiographs of children less than 2 years of age, the thymus may appear to have a straight inferior border on the right side. It looks like the sail of a ship on X-rays and is known as “Sails sign”.

Secondary lymphoid organs are a place where antigen-specific lymphocytes’ clonal expansion takes place. In other words, when an antigen is thought to be present these secondary lymphoid organs increase the number of lymphocytes.

Secondary lymphoid organs include:
·        Lymph nodes
·        Spleen
·        Tonsils
·        Adenoids
·        Peyer’s patches in the small intestine
·        Mucosa-associated lymphoid tissue (MALT)

High yield fact: H. Pylori is associated with gastric MALToma.

Lymph node
·        Lymph nodes are oval or bean shaped encapsulated and trabeculated organs.
·        They are one of the secondary lymphoid organs with many afferent and few efferent lymph vessels.
·        In lymph nodes, primary follicles are dense and dormant; while secondary follicles have pale and active germinal centers.
·        Within a lymph node, B cells localize and proliferate in the follicles while T-cells do so in the paracortex.
·        Medullary cords are the area in the lymph node, which is surrounded by sinuses and contains cords of closely packed lymphocytes and plasma cells.
·        The medullary sinuses (or sinusoids) are vessel-like spaces separating the medullary cords and contain histiocytes (immobile macrophages) and reticular cells.

What are three major functions of lymph nodes?

·        Nonspecific filtration of lymph by macrophages.
·        B- and T-cell storage.

·        Activation of immune response.

Interesting fact: When a person develops an extreme cellular immune response to a viral infection, the paracortex becomes enlarged.

·        The spleen is a critical component of the reticuloendothelial system in hematology and immunology.
·        It is structurally and functionally divided into two “pulp” divisions.
·        The red pulp contains long vascular channels and a fenestrated basement membrane allowing for filtration of red blood cells (RBCs). Older senescent RBCs are filtered into the sinusoids but are unable to reenter the circulation and are phagocytized by splenic macrophages.
·        The white pulp contains the periarterial lymphatic sheath (PALS) that contains T cells and follicles that contain B cells.

What happens after splenectomy?

Moderate thrombocytosis (The spleen can store one third of total body platelets, so removal allows more to circulate in the blood).

·        Howell-Jolly bodies (Nuclear remnants in RBCs).
·        The response to vaccines is poor.

Higher risk for infection by encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Salmonella—“SHiNS”).

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