Structure of the Immune System
High-Yield Summary
- Immune system has 2 branches: innate (fast, nonspecific, no memory) and adaptive (slower on first exposure, specific, forms memory).
- All leukocytes arise from bone marrow stem cells via hematopoiesis.
- B cells mature in the bone marrow; T cells leave the bone marrow to mature in the thymus. The spleen filters blood and initiates immune responses.
- B cells differentiate into plasma cells that produce antibodies (humoral immunity). T cells coordinate responses and directly kill infected cells (cell-mediated immunity).
- Lymph nodes/vessels plus GALT (Peyer's patches), tonsils, adenoids, and appendix all serve immune surveillance.
- Leukocytes split into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Monocytes differentiate into macrophages once they migrate into tissue.
Key Terms
- Hematopoiesis
- Process by which bone marrow stem cells give rise to all leukocyte (white blood cell) types.
- Humoral immunity
- Antibody-mediated immunity carried out by B cells/plasma cells; antibodies circulate in body fluids.
- Cell-mediated immunity
- T-cell-driven immunity involving direct killing of infected cells and coordination of immune responses.
- GALT
- Gut-associated lymphoid tissue, including Peyer's patches in the small intestine — monitors pathogens entering via the gut.
- Macrophage
- Phagocytic cell derived from a monocyte that has migrated into tissue; engulfs pathogens and activates other immune cells.
B Cells vs. T Cells
| B Cells | T Cells |
|---|---|
| Mature in the bone marrow | Mature in the thymus (after leaving bone marrow) |
| Drive humoral immunity | Drive cell-mediated immunity |
| Differentiate into plasma cells → antibodies | Coordinate responses; cytotoxic T cells kill infected cells directly |
Leukocyte Classification
- Neutrophils (granulocyte)
- Most abundant leukocyte; specializes in fighting bacterial infections.
- Eosinophils (granulocyte)
- Combats parasitic infections; involved in allergic responses.
- Basophils (granulocyte)
- Release histamine, drive inflammation and allergic reactions.
- Lymphocytes (agranulocyte)
- B cells (humoral immunity) and T cells (cell-mediated immunity) — key players in adaptive immunity.
- Monocytes (agranulocyte)
- Large phagocytic cells that circulate in blood and differentiate into macrophages in tissue.
Common MCAT Trap
- Both B and T cells originate in bone marrow — only their maturation site differs (bone marrow vs. thymus). Don't say T cells are 'made' in the thymus.
- Granulocytes have visible cytoplasmic granules (neutrophils, eosinophils, basophils); agranulocytes don't (lymphocytes, monocytes) — a monocyte is NOT a granulocyte despite being a large phagocyte.
- The spleen filters blood, not lymph — lymph nodes filter lymph. Don't conflate the two.
Quick Recall
Where do B cells vs. T cells mature?
What are the 2 leukocyte differentiation products of an activated B cell?
What's the difference between granulocytes and agranulocytes?
What does the spleen do?