IB Biology HLDisease & ImmunityPaper 1 & 2~11 min read
White Blood Cells
Two families of white blood cell do almost all the work. Phagocytes eat anything foreign and ask no questions. Lymphocytes are fussy: each one recognises a single antigen and nothing else. Where each cell is made and where it matures is worth learning properly, because it comes up as a straight recall mark.
📘 What you need to know
Phagocytes are white blood cells produced continuously in the bone marrow. They remove dead cells and invasive microorganisms — a non–specific response.
Phagocytes move by amoeboid movement to the site of infection, extend their cell surface membrane around the pathogen and engulf it by endocytosis.
They digest the pathogen with enzymes stored in lysosomes.
Lymphocytes come in two types, T cells and B cells, and drive the specific immune response. They are found in the lymph nodes and circulating in the blood.
T cells are produced in the bone marrow and finish maturing in the thymus. They carry T cell receptors, which have a similar structure to antibodies.
B cells are produced in the bone marrow and mature there too. Their receptors are antibodies, known as antibody receptors.
Each lymphocyte carries a different receptor, so each one binds a different antigen.
Activated lymphocytes divide by mitosis, producing genetically identical clones with the same receptor.
Phagocytes
Phagocytes are made continuously in the bone marrow, so there is always a fresh supply. Their job is to remove dead cells as well as invasive microorganisms, which is a reminder that they are part of general housekeeping and not only defence.
They are part of the innate system, so their response is non–specific: they engulf and digest anything displaying non–self antigens.
Nothing here is specific. The same four steps happen whether the phagocyte meets a bacterium, a fungal spore or a piece of dust.
Two words earn marks in this diagram and students often miss both: endocytosis for how the pathogen gets in, and lysosomes for where the digestive enzymes were stored. Writing “the phagocyte eats it and kills it” scores almost nothing.
Lymphocytes
Lymphocytes are white blood cells too, but they belong to the specific immune response. You will find them in the lymph nodes and circulating in the blood. There are two types, and the letters are a gift: T for thymus, B for bone marrow.
The Y shapes on the surface are the receptors. On a B cell those Y shapes literally are antibodies, which is why B cells can go on to secrete them.
T cells
Also called T lymphocytes. Produced in the bone marrow, they finish maturing in the thymus, which is where the T comes from.
Mature T cells have specific cell surface receptors called T cell receptors, which have a similar structure to antibodies.
Each receptor is specific to one particular type of antigen.
A T cell is activated when it meets and binds its specific antigen on the surface of an antigen–presenting cell. That might be a macrophage, an infected body cell, or the pathogen itself.
Activated T cells then divide by mitosis to increase in number. Mitosis produces genetically identical clones, so every daughter cell carries the same T cell receptor.
B cells
Also called B lymphocytes. They are produced in the bone marrow and remain there while they mature, which is where the B comes from.
B cells have many specific receptors on the cell surface membrane. Those receptors are in fact antibodies, so they are called antibody receptors.
Each B cell has a different type of antibody receptor, so each B cell binds a different antigen.
When a B cell binds its antigen it forms an antigen–antibody complex.
Binding alone is not enough. The B cell is activated by that binding plus cell signalling molecules produced by T helper cells.
Once activated, B cells divide repeatedly by mitosis, producing many clones. Some become plasma cells, which secrete antibodies. Others become memory cells, which stay in the blood.
🧠
T for thymus, B for bone marrow
Both are made in bone marrow. Only the T cell moves out to mature. If you remember where each one matures, you cannot get this wrong.
Phagocyte
Lymphocyte
Part of which system
Innate — non–specific
Adaptive — specific
Made where
Bone marrow, continuously
Bone marrow
Recognises
Any non–self antigen
One specific antigen only
What it does
Engulfs by endocytosis and digests with lysosome enzymes
Divides into clones; makes antibodies or memory cells
Leaves memory?
No
Yes, memory cells remain in the blood
Found where
Blood and at sites of infection
Lymph nodes and circulating in the blood
The link between the two systems. A phagocyte does not just destroy a pathogen. It displays the pathogen’s antigens on its own surface afterwards, which is what turns it into an antigen–presenting cell and lets the adaptive system take over. The innate system is the one that hands the adaptive system its target.
Worked examples
WE 1
Describe how a phagocyte destroys a pathogen
Describe how a phagocyte removes a bacterium from the body. (4 marks)
Point 1: getting there
The phagocyte moves by amoeboid movement to the site of infection and attaches to the pathogen.
Point 2: engulfing
Its cell surface membrane extends out and around the bacterium, taking it in by endocytosis.
Point 3: enclosing
The bacterium is enclosed inside a vesicle within the cytoplasm.
Point 4: digesting
Enzymes stored in lysosomes are released into the vesicle and digest the bacterium.
Move, attach, engulf by endocytosis, digest with lysosome enzymesthis is non–specific, so do not mention antibodies or memory anywhere in the answer
WE 2
Compare T cells and B cells
State two similarities and two differences between T cells and B cells. (4 marks)
Similarities
Both are lymphocytes produced in the bone marrow, and both have receptors on the cell surface membrane that are specific to one antigen.
Difference 1: maturing
T cells finish maturing in the thymus, while B cells mature in the bone marrow.
Difference 2: receptors
T cells carry T cell receptors, which are similar in structure to antibodies. B cell receptors are antibodies.
Same origin and same specificity, different maturing site and receptor“compare” questions need both cells in every point, not a list about one then a list about the other
WE 3
Explain why activated lymphocytes divide by mitosis
Explain the importance of mitosis in the response of an activated lymphocyte. (2 marks)
Point 1: what mitosis produces
Mitosis produces genetically identical clones of the activated cell.
Point 2: why that matters
Every clone therefore carries the same receptor, so all of them recognise the same antigen. This produces a large number of cells all targeting the one pathogen that is actually present.
Identical clones means identical receptors, so the response stays targetedthe mark is for “genetically identical” plus “same receptor” — not just “makes more cells”
💡 Exam tips
Use the technical terms: amoeboid movement, endocytosis, lysosomes.
Say lysosomes store the enzymes. They are not made on the spot.
T for thymus, B for bone marrow — but both are produced in bone marrow.
Say B cell receptors are antibodies, and T cell receptors are similar in structure to antibodies.
Mention that mitosis makes genetically identical clones with the same receptor.
Remember lymphocytes sit in the lymph nodes as well as the blood.
⚠ Common mistakes
Saying T cells are made in the thymus. They are made in the bone marrow and mature in the thymus.
Saying phagocytes are specific. They engulf anything displaying non–self antigens.
Writing “the phagocyte kills it” and stopping. The marks are for endocytosis and lysosome enzymes.
Calling every white blood cell a lymphocyte. Phagocytes are white blood cells too.
Saying B cells make antibodies straight away. They must be activated first, and it is the plasma cells they become that secrete antibodies.
Forgetting the T helper cell. B cell activation needs antigen binding and signalling molecules from T helper cells.
Up next: The Adaptive Immune Response. Now that you know the cells, we put them in order — from a pathogen being engulfed to antibodies flooding the blood, plus the blood group work that sits alongside it.
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