Lymph node structure
Lymph nodes are small, bean-shaped structures distributed throughout the body along the lymphatic vessels. They constitute an integral part of the lymphatic system and possess a distinctive structure, featuring the following elements:
Capsule:
Each lymph node is surrounded by a dense, fibrous connective tissue capsule. This capsule serves as a protective outer covering and provides mechanical support. It helps maintain the shape of the lymph node and acts as a barrier to prevent the spread of pathogens or malignant cells.
Trabeculae:
From the inner surface of the capsule, slender fibrous extensions known as trabeculae project into the lymph node. These trabeculae penetrate deep into the node and divide it into multiple compartments, thereby increasing the surface area for lymph filtration and providing structural scaffolding for the internal components.
Cortex:
The outer part of the lymph node is called the cortex. It contains densely packed lymphoid follicles or nodules. These follicles are rich in B lymphocytes (B cells) and also contain dendritic cells and follicular dendritic cells. In response to antigenic stimulation, some follicles develop lighter-staining regions known as germinal centers, which are sites of active B cell proliferation, differentiation, and antibody production.
Paracortex:
Lying just beneath the cortex and between the cortex and medulla is the paracortical area, often referred to as the paracortex. This zone is predominantly occupied by T lymphocytes (T cells) and is also a major site for antigen presentation by dendritic cells to naïve T cells. The paracortex is essential for T cell activation and proliferation.
Medulla:
The central part of the lymph node is known as the medulla, which contains medullary cords and medullary sinuses. The medullary cords are dense strands composed of lymphocytes (including plasma cells), macrophages, and reticular cells. These are responsible for the final stage of lymph filtration and the secretion of antibodies. The sinuses are open channels that allow lymph to flow and be inspected by immune cells before it exits the node.
Afferent Lymphatic Vessels:
Numerous afferent lymphatic vessels penetrate the capsule on the convex side of the lymph node. These vessels transport lymph fluid into the node from surrounding tissues. The lymph enters the subcapsular sinus, then flows through cortical sinuses and finally into the medullary sinuses.
Efferent Lymphatic Vessels:
After being filtered and processed within the node, lymph exits through a single efferent lymphatic vessel located at the concave side of the node called the hilum. The efferent vessels carry the lymph away toward larger lymphatic ducts and eventually back into the bloodstream.
Functions of Lymph Nodes
Lymph nodes undertake pivotal roles in the body’s immune system, actively participating in several key functions:
- Filtration: Lymph nodes act as filtration units where lymph is cleansed of pathogens, foreign particles, cellular debris, and cancer cells. As lymph circulates through the sinuses, macrophages and dendritic cells within the node trap and phagocytose harmful agents. This filtering process helps to prevent the spread of infections and maintains internal homeostasis.
- Immune Surveillance: Lymph nodes are reservoirs of immune cells, including B cells, T cells, macrophages, dendritic cells, and plasma cells. These immune cells constantly monitor the incoming lymph for signs of infection, foreign antigens, or tumor cells. When foreign substances are detected, the immune cells become activated and initiate appropriate immune responses.
- Activation of Immune Responses: Within the lymph nodes, antigen-presenting cells (APCs), such as dendritic cells, process and present antigenic fragments to naïve T cells in the paracortex and B cells in the follicles. This antigen presentation triggers the activation and clonal expansion of lymphocytes, resulting in a highly specific and adaptive immune response.
- Antibody Production: Following the activation of B cells in the germinal centers, these cells differentiate into plasma cells, which then migrate to the medullary cords. Plasma cells synthesize and secrete large quantities of antibodies, which are specific to the invading antigens. These antibodies enter circulation and neutralize pathogens systemically.
- Cell-Mediated Immunity: T lymphocytes that are activated within the lymph nodes differentiate into various subtypes, including cytotoxic T cells, which directly kill infected or abnormal cells, and helper T cells, which coordinate and amplify the immune response by releasing cytokines.
- Lymphocyte Maturation: Although lymphocyte development begins in primary lymphoid organs (the bone marrow and thymus), the secondary lymphoid organs like lymph nodes serve as critical sites where these cells undergo further maturation, selection, and functional specialization, particularly in response to specific antigens.
- Signal Amplification: Upon the detection of pathogens, lymph nodes become immunologically active sites. Cytokines and chemokines are secreted, attracting more immune cells to the node and amplifying the immune signaling pathways. This enhances the strength and efficiency of the immune response.
In conclusion, lymph nodes are not merely passive structures but are dynamic and responsive centers that are indispensable to the body’s immune surveillance and defense mechanisms. They effectively filter lymph, identify harmful agents, and coordinate both humoral (antibody-mediated) and cell-mediated immune responses. Their intricate structure, composed of a fibrous capsule, cortex, medulla, and network of lymphatic vessels, supports the complex interactions between immune cells necessary for the recognition, processing, and elimination of pathogens. Without lymph nodes, the body’s capacity to mount timely and effective immune responses would be severely compromised.