Germinal centers


Germinal centers are specialized structures within lymphoid tissues where B cells (a type of white blood cell) mature and differentiate. These centers play an important role in the body’s immune response by helping train B cells to produce antibodies that target specific pathogens.

Where are germinal centers normally found?

Germinal centers are typically found within lymphoid tissues such as:

  • Lymph nodes: Small, bean-shaped structures that filter lymph fluid and contain immune cells.
  • Spleen: An organ that filters blood and helps fight infections.
  • Oropharynx: Lymphoid tissue located at the back of the throat.
  • Peyer’s patches: Lymphoid tissue in the lining of the small intestine.

What do germinal centers do?

Germinal centers participate in the normal development and activation of B cells.

Events that take place in a germinal center include:

  • Maturation: B cells develop and mature within germinal centers.
  • Differentiation: B cells differentiate into plasma cells that produce antibodies or memory B cells that provide long-term immunity.
  • Selection: Germinal centers select B cells that produce high-affinity antibodies, ensuring an effective immune response.

What types of cells are normally found in a germinal center?

Germinal centers contain several types of cells, including:

  • B cells: The primary cells that mature and differentiate within the germinal center.
  • T cells: A type of white blood cell that helps regulate the immune response.
  • Follicular dendritic cells: Cells that help present antigens (foreign substances) to B cells and support their maturation.
  • Tingible body macrophages: Macrophages that engulf and digest dead cells within the germinal center.

Types of cells not normally found in a germinal center

Certain types of immune cells are not typically found in germinal centers, and their presence can suggest specific conditions:

  • Eosinophils: These are a type of white blood cell usually involved in allergic reactions and parasitic infections. Their presence in a germinal center can be associated with conditions such as Hodgkin lymphoma and certain parasitic infections.
  • Neutrophils: These cells are typically involved in acute inflammation and bacterial infections. Their presence in germinal centers can indicate acute infection or an inflammatory condition.

What is the appearance of a normal germinal center under the microscope?

Under the microscope, a normal germinal center appears as a well-defined, rounded area within a lymphoid follicle. Follicles are the larger structures within lymphoid tissues that contain germinal centers. They comprise a central germinal center surrounded by a mantle zone of small, resting B cells.

Key features of a normal germinal center:

  • Dark and light zones: Normal germinal centers have distinct dark (centroblast-rich) and light (centrocyte-rich) zones.
  • High mitotic activity: Indicative of active cell division and selection of B cells.
  • Presence of tingible body macrophages: These cells are scattered throughout the germinal center, engulfing dead cells.
The round structure between the white arrows is a germinal center in a lymph node.
The round structure between the white arrows is a germinal center in a lymph node.

What conditions are associated with an increased number of germinal centers?

An increased number of germinal centers can be seen in conditions such as:

  • Reactive lymphoid hyperplasia: An increase in the size and number of lymphoid follicles in response to infection or inflammation.
  • Autoimmune diseases: Conditions where the immune system attacks the body’s own cells, such as rheumatoid arthritis and lupus.
  • Chronic infections: Long-term infections can stimulate the formation of additional germinal centers.

What conditions are associated with abnormal-looking germinal centers?

Abnormal-looking germinal centers can be seen in conditions such as:

  • Follicular lymphoma: A specific type of lymphoma that affects germinal centers and causes them to have a characteristic appearance.
  • Other types of lymphoma: In addition to follicular lymphoma, other types of lymphoma, including diffuse large B cell lymphoma, Hodgkin lymphoma, and Burkitt lymphoma, can cause germinal centers to appear irregular or disrupted.
  • HIV/AIDS: An infection that can lead to abnormal changes in the structure and function of germinal centers.
  • Inflammatory conditions: Conditions like Castleman’s disease can result in abnormal-looking germinal centers.

 

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