Zoology · Ch 8 — Immunology
Cells of the immune system
Cells of the immune system
Blood carries a defined mixture of immune cell types. Lymphocytes make up roughly 20–30% of white blood cells and are recognisable by a large nucleus that fills most of the cell, surrounded by only a thin rim of cytoplasm. Both principal lymphocyte types, B cells and T cells, originate in the bone marrow, but they diverge from there: B lymphocytes remain and mature in the bone marrow itself before circulating — some staying in blood, others settling in lymph nodes and the spleen — while T lymphocytes leave the bone marrow early and complete their maturation in the thymus before joining B cells in the same peripheral tissues.
Both cell types carry antigen-specific receptor proteins on their surface. When a B cell's receptor binds its matching antigen, the B cell activates, proliferates rapidly, and differentiates into antibody-secreting plasma cells; some activated B cells instead become long-lived memory cells, which are what make the secondary immune response so much faster. T lymphocytes take a different route — they never secrete antibodies themselves. Instead, Helper T cells recognise antigen-presenting cells and release chemical messengers called cytokines that activate B cells, while Killer T cells patrol the body and directly destroy cells that are damaged or infected. …
| Cell Type | Number of Cells per µl | Approximate Percentage |
|---|---|---|
| Red blood cells | 4,200,000 – 6,500,000 | – |
| White blood cells — Agranulocytes — Lymphocytes | 1,500 – 4,000 | 20–30 |
| White blood cells — Agranulocytes — Monocytes | 200 – 950 | 2–7 |
| White blood cells — Granulocytes — Neutrophils | 2,000 – 7,000 | 50–70 |
| White blood cells — Granulocytes — Basophils | 50 – 100 | <1 |
What this figure shows. This figure illustrates the major cells of the immune system and their lineage relationships — lymphocytes (B cells maturing toward plasma cells and memory cells; T cells differentiating into helper T cells and killer T cells), along with phagocytic cells such as monocytes/macrophages and neutrophils, and antigen-presenting dendritic cells with their characteristic long membrane extensions — showing how these distinct cell types work togethe …