Lymphoid Organs and Lymph Node Structure

Lymphoid organs include primary and secondary types, with a focus on the lymph nodes.

Primary Lymphoid Organs: Sites for lymphoid cell production (e.g., bone marrow, thymus).
- Bone Marrow: Produces all blood cells, including lymphocytes.
- Thymus: Maturation area for T cells, but does not produce them.
Secondary Lymphoid Organs: Sites for lymphocyte activity and proliferation, not production (e.g., lymph nodes).
- Lymph nodes function as filters that house lymphocytes and facilitate immune responses.

General Structure:
- Vary in size and location throughout the body.
- Surrounded by a tough external fibrous capsule.
- Capsule limits stretching; swelling (due to pathogens) can lead to pain.
Internal Structure:
- Trabeculae: Extensions that divide the lymph node into compartments.
- Main divisions: Cortex (outer) and Medulla (inner).
Cortex:
- Contains lymphatic nodules/follicles with germinal centers (where B cells proliferate).
- Houses T cells and immune cells like macrophages and dendritic cells.
Medulla:
- Stores B cells and plasma cells (derived from B cells) which secrete antibodies.
- Contains medullary cords (extensions with immune cells) and medullary sinuses (spaces for lymph flow).

Lymph enters through afferent lymphatic vessels (more numerous) and exits through a single efferent lymphatic vessel via the hilum.
More afferent vessels than efferent vessels ensure accumulation of lymph, allowing sufficient time for immune cells (B cells, T cells, macrophages) to respond to pathogens.

Lymph nodes play a crucial role in filtering lymph, which can transport pathogens, thus activating the immune response.