Other Haematology Things

Question 1

Notes on lymphoid tissues

Answer 1

**Primary lymphoid organs
**Where lymphocytes are generated
- Bone marrow (B cells and thymic precursors)
- Thymus: developing T cells

**Secondary lymphoid organs
**Where lymphocytes are maintained and immune responses occur
- Lymph nodes, spleen white pulp, MALT

Question 2

Notes on thymus

Answer 2

• Located in mediastinum
• Arises from 3rd or 4th pharyngeal pouch embryologically
• 2 lobes, each with multiple lobules
• Lobes divided into cortex and medulla
• Outer cortex: developing T cells
• Inner medulla: mature T cells

Question 3

Notes on lymph node structures

Answer 3

**Cortex
**T cells surrounding B cells, aggregated in primary follicles (also dendritic cells and macrophages to capture free antigen draining in lymph)
Secondary follicles are those that contain germinal centres
- arise from B cell activation and give rise to plasma cells and memory cells

**Paracortex
**Mostly T cells - recirculating lymphocytes, enter via specified structured termed “high endothelial venules”
Also DCs migrated from epithelial sites - come together to maximise chances of encounter between antigen and responding lymphocyte

**Medulla
**Cords of T, B cells, macrophages and PCs

Innate immune system meets adaptive in lymph nodes
- Resting dendritic cells stationed in tissues as sentinels - skin and mucosa, interstitial sites in other organs (heart, kidney)
- Capture pathogen/antigen using innate receptors (PRRs) - store on surface for B cells, engulf and break into peptides to present on surface with MHC molecules for T cell recognition
- Migrate into lymph nodes in response to chemokine signals
- Activate T and B cells - site of clonal selection

Question 4

Notes on spleen

Answer 4

• Main role is control of blood-borne pathogens
• Free antigen/pathogen in blood is captured by APCs
• Lymphocytes localised to white pulp - red pulp mostly related to red blood cell biology

**White pulp
** Central arteriole
Periarteriolar lymphoid sheath (PALS) - Central T area
B cells surround PALS - secondary follicle - have germinal centre
Marginal zone - at the margin of the B cell area, populated by MZ B cells and macrophages (can rapidly produce antibodies)
Blood borne pathogens/antigen - deposited in MZ, bind to dendritic cells which migrate to PALS (T cells)