### 1. **Capsule and Trabeculae:**
- The spleen is covered by a dense, fibrous capsule made of connective tissue.
- From the capsule, trabeculae extend inward, providing a supportive framework within the spleen.
### 2. **White Pulp:**
- Composed of lymphoid tissue, the white pulp is responsible for immune functions.
- It surrounds central arteries and consists of:
- **Periarteriolar Lymphoid Sheaths (PALS):** Concentrated with T-lymphocytes.
- **Lymphoid Nodules (Follicles):** Contain B-lymphocytes and can develop germinal centers during immune responses.
### 3. **Red Pulp:**
- The red pulp is involved in filtering blood and recycling iron.
- It consists of:
- **Splenic Cords (Cords of Billroth):** Network of reticular fibers and cells, macrophages, lymphocytes, plasma cells, and blood cells.
- **Splenic Sinusoids:** Specialized, wide blood vessels with a discontinuous endothelial lining that allows blood cells to pass between the sinusoids and splenic cords.
### 4. **Marginal Zone:**
- This is the area between the white and red pulp.
- It contains antigen-presenting cells and is involved in trapping and processing antigens from the blood.
### 5. **Vascular Supply:**
- The splenic artery enters the spleen, branches into trabecular arteries, and further into central arteries surrounded by PALS.
- Blood flows from the central arteries into penicillary arterioles and then into the red pulp sinusoids.
### Summary of Functions:
- **Immune Response:** The white pulp produces immune cells (T and B lymphocytes) and facilitates the immune response.
- **Blood Filtration:** The red pulp removes old or damaged red blood cells and pathogens from the blood, recycles iron, and stores platelets.
Understanding these structural components helps explain the spleen's roles in both the immune system and the maintenance of healthy blood cells.