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Immunology

Introduction to Immunology (Part 1)

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Accompanying video lecture on Drbeen.com: https://members.drbeen.com/view/overview-innate-and-acquired-arm-1/rkhSjfCd2lW

Introduction to The Immune System

Our immune system is a major organ system in our body that helps defend against the microorganisms trying to invade us. A majority of the patients presenting to a hospital with a complaint of fever have an underlying infection. This infection may have been caused by bacteria, viruses, fungi or parasites.

   

Categories
Immunology

Immunity and Immune System

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IMMUNITY

Immunity is the capability of our body to defend against the microorganisms including bacteria, viruses, fungi, and parasites.

IMMUNE SYSTEM

All cells, chemicals, and tissues acting together to offer immunity make up the immune system.

  • Examples of the cells are white blood cells, macrophages, neutrophils, etc.
  • Examples of the chemical substances are interleukins, major basic protein, interferon gamma, etc.
  • Examples of the tissues are bone marrow, thymus, lymph nodes, spleen, tonsils, etc.

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Categories
Immunology

Thymus

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Thymus is an encapsulated, bilobed gland located in the anterosuperior mediastinum that sits on the heart.

Derived from the third pharyngeal pouch ectoderm, the thymus consists of a dense cortex and pale medulla. The cortex contains immature T cells, while the inner medulla houses mature T cells and whorled calcifications of epithelial reticular cells called Hassall corpuscles—note that T cells move centrally as they mature within the thymus. It is important to note that the thymus normally involutes or atrophies with age. Young children present with large thymuses up until puberty, after which, it decreases in size.

 

Thymus is anatomically important, as it is the main site of differentiation for T cells, which are initially produced in the bone marrow. T cells are responsible for the cellular aspect of adaptive immunity as their various subsets (CD4+, CD8+) help differentiate B cell antibodies, activate macrophages, and can directly kill virally infected cells.  As T cells move centrally from the cortex to the medulla, they are often presented with antigens via APCs (B cells, dendritic cells, macrophages). Presentation of antigens then allows for thymic selection (positive and negative selection) and fully mature and activated T cells ready for secretion and circulation to secondary lymphatic organs.

 

 

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Categories
Immunology

Lymph Nodes

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Lymph Nodes

We have hundreds of lymph nodes in our body. These bean-shaped structures are secondary lymphoid organs. Remember that bone marrow and thymus are the primary lymphoid organs.

Important Functions

A lymph node is a cluster of various kinds of cells. These cells, in turn, perform various functions that are attributed to the lymph node:

  • Macrophages that perform non-specific filtration of the lymph. Macrophages are part of the innate arm of the immune system hence their function is non-specific.
  • B and T cells. These cells are part of the acquired arm of the immune system.
  • Follicular dendritic cells that help refine the B cell’s function.

Structure

Lymph nodes are encapsulated. Their outer region is called cortex. Cortex is covered by a fibrous capsule that sends extensions inwards. These extensions are called trabeculae. Region internal to the cortex is called medulla.

AFFERENT AND EFFERENT CHANNELS

There are many afferent channels bringing lymph from tissues to a lymph node, and efferent channel that brings the lymph back to the heart via the thoracic duct.

CORTEX

OUTER CORTEX

The outer region is called superficial cortex. It contains B cell follicles.

INNER CORTEX

The inner region of the cortex comes in contact with medulla and is called paracortex. Paracortex contains T cells.

FOLLICLES

The superficial cortex contains B cell clusters. These clusters are called follicles.

 

PRIMARY FOLLICLES

Clusters of densely packed inactive B cells appear dark under a microscope. These clusters are called primary follicles.

SECONDARY FOLLICLES

During the infections that activate B cells, the active B cells in the center of the follicles appear relatively light. Follicles with pale germinal centers are called secondary follicle.

MANTEL ZONE

This is the outer part of a secondary follicle. It contains small lymphocytes wrapping around an active germinal center. This is the area of lymphoma in mantle cell lymphoma.

Affinity maturation

In the follicles, special dendritic cells are also present. These follicular dendritic cells present antigens to B cells. This activity allows B cells to refine their antigen binding sites. This process of immune response maturity is called affinity maturation.

Paracortex

This region is sandwiched between the superficial cortex and medulla. It contains T cells. This region is unique that it has high endothelial venules. These venules allow T and B cells from the blood to enter the lymph node tissue.
The paracortical region is less developed in DiGeorge Syndrome.
During the active infections (usually viral) the paracortical region swells due to extreme T cell proliferation.

Medulla

Consists of cords that contain T cells and activated B cells (plasma cells). There are spaces between the cords called medullary sinuses. These sinuses are lined with reticular cells and macrophages. Lymph from these sinuses drains into the efferent lymph channels to be sent back to the cardiovascular system.

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