Two families of white blood cell do the work. Phagocytes are the general clean-up crew that swallow anything foreign. Lymphocytes are the specialists, each one tuned to a single antigen it may never meet. This page is about who does what.
📘 What you need to know
Phagocytes are produced continuously in the bone marrow and remove dead cells and invading microorganisms — a non-specific response.
They move by amoeboid movement to the site of infection and attach to pathogens.
The cell surface membrane extends around the pathogen, engulfing it by endocytosis.
The pathogen is digested by enzymes stored in lysosomes.
Lymphocytes come in two types, T cells and B cells, and carry out the specific immune response.
T cells are made in the bone marrow and mature in the thymus. Mature T cells carry T cell receptors, each specific to one antigen.
B cells mature in the bone marrow. Their receptors are antibodies, and each B cell has a different one.
Lymphocytes are found in the lymph nodes and circulating in the blood.
Phagocytes: eat first, ask nothing
A phagocyte does not care what it has found. If something displays non-self antigens, it gets engulfed. That is the whole strategy, and it is remarkably effective.
Phagocytes also clear away your own dead cells, which is why they are described as removing dead cells as well as invading microorganisms.
🧩 Phagocytosis, step by step
The phagocyte moves to the site of infection by amoeboid movement.
It attaches to the pathogen, which is recognised by its non-self antigens.
The cell surface membrane extends out and around the pathogen, engulfing it by endocytosis.
The pathogen is now enclosed in a vesicle inside the cytoplasm.
Lysosomes fuse with the vesicle and release digestive enzymes, which break the pathogen down.
Phagocytes do one more thing that becomes very important on the next page: after engulfing a pathogen they display its antigens on their own surface membrane. That turns them into an antigen-presenting cell and connects the innate system to the adaptive one.
Lymphocytes: one cell, one antigen
Lymphocytes are white blood cells involved in the specific immune response. They are found in the lymph nodes and circulating in the blood. There are two types, and the letters give away where each one matures.
This is the idea that makes the whole adaptive system work. The variety is generated first, at random, and the pathogen simply picks out the cell that happens to fit.
T cells
T cells, or T lymphocytes, are produced in the bone marrow and finish maturing in the thymus — hence the T. Immature T cells first divide by mitosis in the bone marrow; then, in the thymus, each cell produces T cell receptors and inserts them into its cell surface membrane.
These receptors are similar in structure to antibodies, and each one is specific to a particular type of antigen. A T cell is activated when it meets and binds to its specific antigen displayed on the surface of an antigen-presenting cell — which might be a macrophage, an infected body cell, or the pathogen itself.
Once activated, T cells divide by mitosis. Because mitosis produces genetically identical cells, every daughter cell carries the same type of T cell receptor.
B cells
B cells, or B lymphocytes, remain in the bone marrow as they mature — hence the B. Their receptors are antibodies, embedded in the cell surface membrane, and are known as antibody receptors.
Each B cell carries a different type of antibody receptor, so each one can bind a different type of antigen. Mature B cells then circulate and concentrate in the liver and spleen.
Memory hook.T for thymus, B for bone marrow. It genuinely is that simple, and it is asked often enough to be worth the ten seconds it takes to learn.
Worked examples
WORKED EXAMPLE
Describe how a phagocyte destroys a bacterium. [3]
Getting there and attachingThe phagocyte moves by amoeboid movement to the site of infection and attaches to the bacterium.EngulfingIts cell surface membrane extends around the bacterium, engulfing it by endocytosis into a vesicle.DigestingLysosomes release digestive enzymes into the vesicle, breaking the bacterium down.Attach → engulf by endocytosis → digest with lysosome enzymes“endocytosis” and “lysosome” are the two technical words being marked
WORKED EXAMPLE
Explain why it is an advantage that each lymphocyte has a receptor for a different antigen. [3]
The varietyBetween them, the population of lymphocytes carries an enormous range of different receptors.What that means for a new pathogenWhatever antigen enters the body, there is likely to be at least one lymphocyte whose receptor is complementary to it.The consequenceThat cell can be activated and cloned, so the body can respond specifically to pathogens it has never met before.Variety first, selection secondthis is selection, not design — the receptor is not made to fit the pathogen
💡 Exam tip
Say engulfs by endocytosis, not “eats” or “absorbs”.
Name lysosomes as the source of the digestive enzymes. “Enzymes in the cytoplasm” loses the mark.
Learn where each cell type matures — thymus for T, bone marrow for B — and that both originate in the bone marrow.
Describe a receptor as specific or complementary to one antigen. Both words are accepted.
Remember mitosis produces genetically identical clones, so all daughter cells carry the same receptor.
⚠ Common mix-up
Saying phagocytes are specific. They are not — they engulf anything showing non-self antigens.
Thinking B cells mature in the blood. They mature in the bone marrow.
Confusing an antibody receptor with a free antibody. On a B cell the antibody is anchored in the membrane; plasma cells later release antibodies into the blood.
Writing that a lymphocyte “makes” a receptor to fit the pathogen. The receptors already exist; the pathogen selects the matching cell.
Using “white blood cell” when the question wants a specific name. Say phagocyte, T cell or B cell.
Up next: The Adaptive Immune Response — how one selected B cell turns into millions of antibody factories, and why the second infection feels like nothing at all.
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