Lymph Cells and Tissues

Lymphoid Cells

A lymphocyte is a type of white blood cell in the vertebrate immune system.

Learning Objectives

List the types of lymphocytes in the vertebrate immune system and their roles in innate or humoral immunity

Key Takeaways

Key Points

  • A lymphocyte is a type of white blood cell in the vertebrate immune system.
  • NK cells are a part of the innate immune system and play a major role in defending the host from both tumors and virally infected cells.
  • T cells are involved in cell-mediated immunity whereas B cells are primarily responsible for humoral immunity (relating to antibodies ).
  • Helper T-cells coordinate immune responses, while cytotoxic T-cells lyse (break down) pathogens associated with T cell’s specific antigen.
  • Memory B cells are formed at the end of an adaptive immune response and will produce antibodies more quickly when the antigen is detected again, which is effective at preventing recurrent infections from the same pathogen.
  • Sometimes the body will present antigens that aren’t harmful (allergy) or antigens from otherwise normally functioning body parts (autoimmunity). The latter can cause severe antibody and T-cell induced immune responses and diseases.

Key Terms

  • humoral immunity: Immunity to infection due to antibodies that circulate in the blood and lymph and are produced by B cells.
  • antigen: Any molecule that activates an immune response from a host organism, such as a toxin produced by bacteria or a molecule expressed on the cell wall of a virus -infected cell.
  • lymphocyte: A type of white blood cell that includes T cells, B cells, and NK cells.

A lymphocyte is a type of white blood cell in the immune system. Lymphocytes develop from lymphoblasts (differentiated blood stem cells) within lymphoid tissue in organs such as the thymus. Lymphocytes are vital for normal immune system function. The three major types of lymphocyte are T cells, B cells, and natural killer cells.

Natural Killer Cells

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Lymphocyte: A stained lymphocyte surrounded by red blood cells viewed using a light microscope.

Natural killer (NK) cells are part of the innate immune system and play a major role in defending the host from both tumors and virus-infected cells. NK cells contain receptors for a molecule called MHC (major histocompatibility complex) class I, which allows the NK cell to distinguish between infected cells and tumors from normal and uninfected cells. Normal cells express MHC class I on their cell membranes, while infected or cancerous cells do not express or express reduced amounts of the molecule. Therefore, the molecule acts as an inhibitor of NK cell activity, and NK cells activate and destroy cells on which MHC class I is not detected.

Activated NK cells release cytotoxic (cell-killing) granules that contain perforin and granzyme, which can lyse (break down) cell membranes and induce apoptosis to kill infected or abnormal cells. Cancer cells express much less MHC class I than normal cells, so NK cells are effective at destroying them before they develop into full tumors. If cancer cells evade NK cell detection for long enough, however, they can grow into tumors that are more resistant to NK cell activity.

T Cells and B Cells

T and B lymphocytes are the main forces of adaptive immunity, which includes cell-mediated and humoral immunity. T cells are involved in cell-mediated immunity whereas B cells are primarily responsible for humoral immunity. T cells and B cells irecognize specific “non-self” antigens during a process known as antigen presentation with MHC class II (usually done by dendritic cells). Once they have received an antigen, the cells become specifically tailored to eliminate and inhibit the pathogens or pathogen-infected cells that express that antigen. Sometimes these lymphocytes react to antigens that aren’t harmful (allergy) or will attack antigens expressed from the host’s own body (autoimmunity).

There are two types of T cells involved in adaptive, cell-mediated immunity.

  • Helper T cells (CD4s) facilitate the organization of immune responses. They present antigens to B cells, produce cytokines that guide cytotoxic T cells, and activate macrophages.
  • Cytotoxic T cells (CD8s) destroy pathogens associated with an antigen. Similar to NK cells, they bind to MHC class I and release granzymes, but will only bind to cells that express their specific antigen. Cytoxic T cells cause much of the damage associated with cell-mediated hypersensitivity, autoimmune disorders, and organ transplant rejection.

B cells are part of the humoral component of adaptive immunity. They respond to pathogens by producing large quantities of antigen-specific antibodies which neutralize foreign objects like bacteria and viruses, and opsonize (mark) them to be more easily recognized by other immune cells.

Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered in the form of memory cells. Memory B cells are important for quickly producing antibodies should an antigen be recognized again, which can prevent recurrent infections from the same type of pathogen. This explains why vaccines are so effective, though viruses and bacteria with high mutation rates will express different antigens and thus avoid recognition by memory cells.

Development of Lymphocytes

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SEM Lymphocyte: A scanning electron microscope (SEM) image of a single human lymphocyte.

All lymphocytes originate from a common lymphoid progenitor cell known as a lymphoblast, before differentiating into their distinct lymphocyte types. The formation of lymphocytes is known as lymphopoiesis. B cells mature into B lymphocytes in the bone marrow, while T cells migrate to and mature in thymus. Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs, where they survey for invading pathogens and cancer cells. The lymphocytes involved in adaptive immunity (B and T cells) differentiate further after exposure to an antigen, which occurs in the lymph nodes during antigen presentation from the dendritic cells. The fully differentiated B and T cells are specific to the presented antigen and work to defend the body against pathogens associated with that antigen.

Lymphoid Tissue

Lymphoid tissue consists of many organs that play a role in the production and maturation of lymphocytes in the immune response.

Learning Objectives

Describe the structure, development, and location of lymphoid tissue

Key Takeaways

Key Points

  • Lymphoid tissue may be primary or secondary depending upon its stage of lymphocyte development and maturation.
  • The secondary lymphoid tissues consist of lymph nodes, tonsils, Peyer’s patches, spleen, adenoids, skin, and mucosa-associated lymphoid tissue (MALT). They are responsible for maintaining mature naive lymphocytes and initiating an adaptive immune response.
  • The thymus and bone marrow constitute the primary lymphoid tissues that are the sites of lymphocyte generation and maturation.
  • Lymphoid tissue develops from venous endothelial tissues after the fifth week of gestation, starting by the end of the lymphatic system (subclavian vein and lymph ducts) and spreading outwards.

Key Terms

  • secondary lymphoid organ: These organs maintain mature naive lymphocytes and initiate an adaptive immune response through antigen presentation.
  • primary lymphoid organ: These organs generate lymphocytes from immature progenitor cells and provide an environment in which they mature.

The tissues of lymphoid organs are different than the tissues in most other organ systems in that they vary considerably based on cell cycle proliferation of lymphocytes. The lymphoid tissue may be primary or secondary depending upon its stage of lymphocyte development and maturation. Specialized lymphoid tissue supports proliferation and differentiation of lymphocytes.

Primary Lymphoid Organs

Central or primary lymphoid organs generate lymphocytes from immature progenitor cells such as lymphoblasts. The thymus gland and bone marrow contain primary lymphoid tissue where B and T cells are generated.

Besides generation, primary lymphoid tissue is the site where lymphocytes undergo the early stages of maturation. T cells mature in the thymus, while B cells mature in the bone marrow. T cells born in bone marrow travel to the thymus gland to mature.

Secondary Lymphoid Organs

Secondary or peripheral lymphoid organs maintain mature naive lymphocytes until an adaptive immune response is initiated. During antigen presentation, such as from the dendritic cells, lymphocytes migrate to germinal centers of the secondary lymphoid tissues, where they undergo clonal expansion and affinity maturation. Mature lymphocytes ill then recirculate between the blood and peripheral lymphoid organs until they encounter the specific antigens where they perform their immune response functions.

Secondary lymphoid tissue provides the environment for the antigens to interact with the lymphocytes. It is found mainly in the lymph nodes, but also in the lymphoid follicles in tonsils, Peyer’s patches, spleen, adenoids, skin, and other areas associated with the mucosa-associated lymphoid tissue (MALT). In addition to supporting B and T lymphocyte activation, other secondary lymphoid organs perform other unique functions, such as the spleen’s ability to filter blood and the tonsil’s ability to capture antigens in the upper respiratory tract.

This diagram of lymphatic tissue indicates the cervical lymph nodes, lymphatics of the mammary gland, cisterna chyli, lumbar and pelvic lymph nodes, lymphatics of the lower limbs, inguinal lymph nodes, lymphatics of the upper limbs, spleen, axillary lymph nodes, thoracic duct, and thymus.

Lymphatic Tissues: The thymus and bone marrow are primary lymphoid tissue, while the lymph nodes, tonsils, and spleen are secondary lymphoid tissue.

Development of Lymphatic Tissue

Lymphatic tissue begins to develop by the end of the fifth week of embryonic development. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm, the inner tissue layer of the embryo. Development of lymphatic tissue starts when venous endothelial tissues differentiate into lymphatic endothelial tissues. The lymphatic endothelial cells proliferate into sacs that eventually become lymph nodes, with afferent and efferent vessels that flow out from the lymph nodes. This process begins with he lymph nodes closest to the thoracic and right lymph ducts, which arises from immature subclavian-jugular vein junction. The lymph nodes organized around other lymph trunks, such as those in the abdomen and intestine, develop afterwards from nearby veins. Smaller lymph vessels and lymphatic capillaries develop after that until the lymphatic system is completed at the closed end of each lymphatic capillary.

More specialized primary lymph tissue, such as the thymus, develops from pharyngeal pouches (embryonic structures that differentiate into organs near the pharynx and throat) by the eighth week of gestation.

Lymph Nodes

Lymph nodes are small oval-shaped balls of lymphatic tissue distributed widely throughout the body and linked by lymphatic vessels.

Learning Objectives

Describe the structure and function of lymph nodes

Key Takeaways

Key Points

  • Lymph nodes are well-distributed around the chest, armpits, neck, and abdomen.
  • Each lymph node is surrounded by a fibrous capsule that encircles the internal cortex and medulla. The cortex is mainly composed of clusters of B and T cells. The medulla contains plasma cells, macrophages, and B cells, as well as sinuses, which are vessel-like spaces that the lymph flows into, and nodules located within the sinuses.
  • Lymph nodes contain a hilum beneath the capsule, which brings blood supply to the tissues of the lymph node.
  • Antigen presentation by dendritic cells occurs in the lymph nodes, which triggers an adaptive immune response.
  • Lymphadenopathy, the swelling of the lymph nodes, can indicate the presence of an infection or cancer.
  • Lymph circulates to the lymph node via afferent lymphatic vessels and drains into the efferent lymphatic vessels just beneath the capsule.

Key Terms

  • lymphadenopathy: Swelling of the lymph nodes that can indicate the presence of an infection or cancer.
  • lymph node: Small oval bodies of the lymphatic system that act as filters, with an internal honeycomb of connective tissue filled with lymphocytes and macrophages that collect and destroy bacteria, viruses, and foreign matter from lymph.

Lymph nodes are small oval-shaped balls of lymphatic tissue, distributed widely throughout the body and linked by a vast network of lymphatic vessels. Lymph nodes are repositories of B cells, T cells, and other immune system cells, such as dendritic cells and macrophages. They act as filters for foreign particles in the body and are one of the sites where adaptive immune responses are triggered.

Structure of Lymph Nodes

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Lymph node structure: This diagram of a lymph node shows the outer capsule, cortex, medulla, hilum, sinus, valve to prevent backflow, nodule, and afferent and efferent vessels.

Lymph nodes are found throughout the body, and are typically 1 to 2 centimeters long. Humans have approximately 500–600 lymph nodes, with clusters found in the underarms, groin, neck, chest, and abdomen. Each lymph node is surrounded by a fibrous capsule that encircles the internal cortex and medulla. The cortex is mainly composed of clusters of B cells in the outer layers and T cells in the inner layers, and may also contain antigen-presenting dendritic cells. The medulla contains plasma cells, macrophages, and B cells as well as sinuses, which are vessel-like spaces that the lymph flows into. Inside each sinus cavity is a nodule, a smaller, denser bundle of lymphoid tissue that usually contains a germinal center, the site of B cell proliferation during antigen presentation. The sinuses are partially divided by capsule tissue, which causes lymph fluid to flow around the nodules in each sinus cavity on their way through the node.

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The lymphatic system: This diagram shows the network of lymph nodes and connecting lymphatic vessels in the human body.

Lymph fluid flows into and out of the lymph nodes via the lymphatic vessels, a network of valved vessels that are similar in structure to cardiovascular veins. Each lymph node has an afferent lymph vessel that directs lymph into the node, and an efferent lymph vessel called the hilum that directs lymph out of the node at the concave side of the node. The hilum also contains the blood supply of the lymph node.

Function of Lymph Nodes

Lymph nodes are the primary site for antigen presentation and activation in adaptive immune response in B and T lymphocytes. These lymphocytes are continuously recirculated through the lymph nodes and the bloodstream. Molecules called antigens are found on bacteria cell walls, the cell walls of virus-infected cells, or even chemical substances and toxins secreted from bacteria. These antigens may be taken by cells into the lymph nodes. There, antigen-presenting cells called dendritic cells present the antigen molecule to naive B and T lymphocytes. These undergo cell cycle proliferation into lymphocytes that are able to specifically detect and eliminate pathogens associated with that antigen, through various methods such as cytotoxic action (T cells) and antibody production (B cells).

The lymph nodes also filter the lymph fluid. Macrophages in the sinus spaces phagocytize (engulf) foreign particles such as pathogen, so that lymph fluid that returns to the bloodstream is cleaned of problematic abnormalities. The lymph node is also arranged in such a way that the chance of B and T lymphocytes encountering dendritic cells is quite high, to facilitate antigen presentation.

Lymphadenopathy

Lymphadenopathy describes the clinical condition of swollen lymph nodes. This is usually caused by increased lymph flow into the nodes. This fluid may carry a higher amount of debris, so inflammation occurs as more neutrophils and later macrophages enter the node to remove debris from the lymph.

Lymphadenopathy is a symptom in conditions from trivial, such as a common cold or a minndor infection, to life-threatening, such as cancer or severe infection. Cancers that are severe and widespread from frequent metastases tend to have lymphadenopathy, so cancer staging criteria includes lymph node involvement. Additionally, cancers like lymphomas that have tumors made out of aberrant lymphocytes nearly always show lymphadenopathy, often an early warning sign for this type of cancer.