White Blood Cells & Immune system

Some diseases are caused by microorganisms that get inside the body. A microorganism that causes disease is called a pathogen. Examples include some bacteria and viruses.

The body has a defence system called the immune system. A key part of it is found in the blood: the white blood cells.

• White blood cells are made in the bone marrow and travel around the body in the blood.
• Their job is to find and destroy pathogens that have got into the body.
• For this subtopic you need the two types of white blood cell named in the specification: phagocytes and lymphocytes.

Each type defends the body in a different way, and a good exam answer must clearly distinguish between them.

Exam tip. The keyword is pathogen (a disease-causing microorganism) — not just "germ" or "bug". Saying white blood cells "fight illness" is too vague; say they destroy pathogens.

The first type of white blood cell is the phagocyte.

A phagocyte destroys pathogens by ingesting (engulfing/swallowing) and then digesting them. This whole process is called phagocytosis.

How it works, step by step:

1. A phagocyte detects a pathogen (such as a bacterium) and moves towards it.
2. The phagocyte changes shape and flows around the pathogen, engulfing it — taking it inside the cell.
3. The pathogen is now trapped inside the phagocyte; enzymes are released onto it.
4. The enzymes digest (break down) the pathogen, destroying it.

A useful way to remember it: "phagocyte = eats the pathogen".

Exam tip. The two scoring words for a phagocyte are engulf/ingest and digest. The whole process is named phagocytosis — learn that spelling.

The second type of white blood cell is the lymphocyte.

Lymphocytes do not engulf pathogens. Instead, they release antibodies into the blood.

• An antibody is a protein made by a lymphocyte.
• Antibodies attach to pathogens and help to destroy them.

The crucial idea is that antibodies are specific:

• Every pathogen carries marker molecules on its surface called antigens.
• A lymphocyte makes an antibody with a shape that matches one particular antigen — like a key fits one lock.
• That antibody will only attach to a pathogen with that antigen, and to no other.

So a different antibody is needed for each different pathogen. This is why, for example, the antibody that protects you against measles does not protect you against flu — the antigens are different shapes.

Exam tip. Link the keywords together: lymphocyte → releases antibody → antibody is specific to one antigen on a pathogen. The word specific (and the idea of a matching shape) is the high-value point examiners look for.

It helps to picture how an antibody recognises a pathogen.

• A pathogen has antigens on its surface — molecules the body recognises as foreign (not its own).
• A lymphocyte makes an antibody with a complementary shape to that antigen.
• The antibody binds to the antigen, marking the pathogen for destruction.

Putting the two cell types together, the immune system response to a pathogen uses both:

• Phagocytes engulf and digest pathogens (a general response).
• Lymphocytes release specific antibodies that target a particular pathogen by its antigen.

Exam tip. Do not muddle the words. Antigen = the marker on the pathogen. Antibody = the protein made by the lymphocyte that matches it. They sound alike but mean opposite things.

The specification asks you to distinguish the two white blood cell types. Learn this contrast clearly:

	Phagocyte	Lymphocyte
What it does	Engulfs (ingests) and digests pathogens	Releases antibodies
Name of action	Phagocytosis	Antibody production
General or specific?	General — engulfs many types of pathogen	Specific — each antibody matches one antigen
Key word	engulf / digest	antibody / antigen / specific

A simple memory pair:

• Phagocyte → "phago" = eating → it eats (engulfs) pathogens.
• Lymphocyte → makes antibodies that lock onto one antigen.

Exam tip. A question that asks you to "compare" or "distinguish" the two cells wants you to say what each one does. A common error is to describe phagocytes for both cells. Always pair lymphocyte with antibody and specific.