What are lymph nodes?

These are about 100-200 100 bean shaped organs, which are found along lymphatic vessels, and which filter micro-organisms etc from lymph.

This is a diagram of a lymph node, cut away to show the organisation inside, into cortical and medullary regions.

Primary follicles: lymphoid follicles without a germinal centre.

Secondary follicles: lymphoid follicles with a germinal centre. These mostly contain B-cells.

The nodes are covered by a capsule of dense connective tissue, and have capsular extensions, of connective tissue, called the trabeculae, which provide support for blood vessels entering into the nodes.
Lymph, containing micro-organisms, soluble antigens, antigen presenting cells, and a few B-cells, enters the lymph node via afferent lymphatic vessels which enter the subcapsular sinus. It then runs through cortical sinuses into medullary sinuses and leaves through the efferent lymphatic vessels, at the Hilium as efferent lymph. This contains lots of T-lymphocytes, B-lymphocytes, plasma cells and antibody.
All the blood sinuses are lined by a discontinuous layer of simple squamous endothelium, and they also contain lymphocytes and macrophages. Reticular fibres provide additional support to the matrix/stroma.
The cortex is divided into an outer and an inner cortex.

The outer cortex has lymphatic nodules that mostly contain B-cells. Small lymphocytes sit in the spaces between the reticular fibre meshwork in the cortex. (see the picture below).
The lighter staining areas are germinal centres, where the B-cells proliferate into antibody secreting plasma cells (see B-and T-lymphocytes). Macrophages are also present in these regions, together with dendritic cells, and some T-cells. Both the macrophages, and the dendritic cells trap antigens and present them on their surfaces to B-cells.

The inner cortex contains mostly T-cells.

The deep cortical, and medullary cords contain B-cells and plasma cells.Plasma cells live for 3 days, and make IgG type antibodies.

This diagram of a lymph node shows the pathways that lymphocytes can take, in and out of the lymph node.

Most of the lymphocytes enter the lymph nodes via blood vessels, and about 10% enter through the lymph.

The structure of the post-capillary venule, in the deep cortex (paracortex) is unusual in that it is not lined by simple squamous epithelium, but by a simple cuboidal epithelium. These are called high endothelial venules (HEVs) (see the picture below). Lymphocytes recognise and adhere to these endothelial cells, and squeeze through them into the deep cortical regions of the nodes. This region of the lymph has lots of T-cells, as well as the antigen presenting dendritic cells.

T-cells entering here become activated in the cortex, between lymphoid follicles.