Antigens and antibodies
An antigen is a “name tag” that the immune system recognises, and an antibody is a “key” that fits that tag exactly. These two ideas underpin immunity, vaccines, blood tests and many modern leukemia treatments. This topic explains what they are, how they work and what they mean for a person with leukemia.
01Short answer
02Simple explanation
An antigen is something the immune system “recognises”, such as a protein on the surface of a virus. Think of it as a person’s face: the immune system looks at the face and decides whether it belongs to a “friend” or a “stranger”.
An antibody is a small protein shaped like the letter Y. Its two “arms” fit one particular antigen exactly, like a key fits a lock. When it meets the matching antigen, it holds on tightly.
When an antibody sticks to a germ, the germ cannot get into cells. In addition, a germ coated in antibodies is easier for macrophages and neutrophils to recognise and swallow.
Antibodies are made by plasma cells. They form when matching B cells multiply and change. The first time, this takes several days; the next time it is faster thanks to memory cells.
The link with leukemia: doctors use lab-made antibodies to find antigens on the surface of leukemia cells. Some medicines are antibodies too: they attach to an antigen on a leukemia cell and tag it for the immune system.
03Scientific explanation
Definition: according to MedlinePlus, an antigen is any substance that causes the immune system to make antibodies against it; it can come from the environment or form inside the body. According to MedlinePlus, each type of antibody defends against one specific antigen; when the immune system mistakes healthy tissue for something harmful, an autoimmune disorder results.
The three jobs of antibodies: according to InformedHealth, antibodies make germs harmless directly (by attaching to the surface of viruses or bacteria or to their toxins), activate other immune cells and make the work of phagocytes easier, and switch on defence proteins. In this way, adaptive immunity helps innate immunity.
Classes of antibody: according to Chaplin’s review, IgM is made first in a response, IgG is the most plentiful class in the blood and provides long-term protection, IgA works at the mucous membranes, and IgE is involved in allergic reactions and defence against parasites. According to MedlinePlus, the normal IgG level in adults is about 700–1600 mg/dL.
Monoclonal antibodies: according to the NCI, these are lab-made immune proteins that recognise a specific target. For example, rituximab binds to the CD20 protein on B cells, causing the immune system to kill them; blinatumomab binds to both CD19 on leukemia cells and CD3 on T cells at the same time.
04What happens in the body?
Our cells’ own antigens
Every cell carries proteins on its surface that show what type of cell it is; doctors call them “CD markers”. According to the NCI, immunophenotyping uses lab-made antibodies to find these markers. This shows whether a leukemia cell belongs to the B, T or myeloid line, and is later used to track residual disease during treatment.
What do antibody tests show?
Finding certain antibodies in the blood can show that a person has met that germ in the past or has been vaccinated. For example, ASCO recommends testing for hepatitis B before treatment, for both the antigen and antibodies, because a past infection can become active again during treatment.
When antibody levels fall
According to the NCI, a low level of immunoglobulins in the blood is called hypogammaglobulinemia and raises the risk of infection. It is common in CLL and can also develop after some treatments. According to the iwCLL guidelines, if serious infections keep coming back, a doctor may consider immunoglobulin replacement therapy.
Blood groups are antigens too
According to the American Red Cross, blood types are defined by the presence or absence of certain antigens on the surface of red blood cells. If incompatible blood is transfused, the recipient’s antibodies can attack it. That is why blood group and compatibility are checked before every transfusion.
How do monoclonal antibodies work?
A lab-made antibody attaches to a specific antigen on the surface of a leukemia cell. It can then tag the cell for the immune system, bring T cells close to it or deliver a drug to it. If the target antigen is also present on healthy cells, they can be affected as well.
05From antigen to antibody
A simplified sequence.
06The main antibody classes
| Class | Where it is found | Main role |
|---|---|---|
| IgG | Blood and tissue fluid (most plentiful) | Long-term protection, tagging germs |
| IgM | Blood | The first antibody made at a first encounter |
| IgA | Mucous membranes, saliva, breast milk | Protecting the mucous membranes |
| IgE | Small amounts, in the tissues | Allergic reactions, defence against parasites |
| IgD | On the surface of B cells | Involved in activating B cells |
* This table is simplified. Normal antibody levels depend on the laboratory.
07Key terms
- Antigen
- Any substance that triggers an immune response.
- Antibody
- A protein made by plasma cells against a specific antigen.
- Immunoglobulin
- The scientific name for antibodies; the classes are IgG, IgA, IgM, IgE and IgD.
- Plasma cell
- A cell derived from a B cell that makes large amounts of antibody.
- CD marker
- A protein on a cell’s surface that shows its type.
- Monoclonal antibody
- A lab-made antibody that recognises one target.
- Hypogammaglobulinemia
- A low level of antibodies in the blood.
- Autoimmune disease
- When the immune system mistakenly attacks the body’s own tissues.
- Serology test
- A test that looks for antibodies or antigens in the blood.
- Immunophenotyping
- A lab method that uses antibodies to find cell markers.
08Key points
- An antigen is a “name tag” that the immune system recognises.
- An antibody is a Y-shaped protein that fits an antigen like a key in a lock.
- Antibodies are made by plasma cells, which develop from B cells.
- Antibodies make germs harmless and tag them.
- There are five main antibody classes; IgG is the most plentiful in the blood.
- Markers on leukemia cells are used in diagnosis.
- Monoclonal antibodies are used to treat some leukemias.
- Your team decides on infection protection when antibody levels are low.
09Frequently asked questions
How can I remember the difference between antigen and antibody?
The antigen is the “target” and the antibody is the “key” that fits it. The antigen sits on a germ or a cell; the antibody is the response made by the body (or by a lab).
Do I need my antibody levels checked?
That depends on the type of disease and treatment. For example, in CLL or after some treatments, a doctor may check immunoglobulin levels. Your team decides when this is needed.
Are monoclonal antibodies chemotherapy?
No. According to the NCI, they are a type of targeted therapy, and some are also immunotherapy. Their side effects differ from chemotherapy but can occur, such as infusion reactions and a higher risk of infection.
Can my test results change after an antibody medicine?
Yes, in some cases. For example, after medicines that target B cells, your own antibody production can fall for a while. Discuss the timing of vaccines and tests with your team.
Does a blood transfusion change my antibodies?
Because blood group compatibility is checked, transfusions are usually safe. After many transfusions, new antibodies can sometimes form; that is why the blood bank rechecks compatibility every time.
Can antibodies protect against leukemia?
Your own antibodies do not usually prevent leukemia, because leukemia cells come from your own cells. Lab-made antibodies, however, help in treatment by targeting specific leukemia markers.
10Questions for your doctor
Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.
Are my immunoglobulin (antibody) levels low?
If antibodies are low, do I need immunoglobulin infusions?
Does my treatment affect antibody production?
How would you explain my antibody test results?
12Sources
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: antigen
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: antibody
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: immunoglobulin
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: hypogammaglobulinemia
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: immunophenotyping
- MedlinePlus (U.S. National Library of Medicine). Antigen
- MedlinePlus (U.S. National Library of Medicine). Antibody
- MedlinePlus (U.S. National Library of Medicine). Quantitative nephelometry test
- InformedHealth.org (IQWiG). The innate and adaptive immune systems
- Journal of Allergy and Clinical Immunology, 2010. Chaplin DD. Overview of the immune response
- National Cancer Institute (NCI). Monoclonal Antibodies
- Blood, 2018. iwCLL guidelines for diagnosis, indications for treatment, response assessment, and supportive management of CLL
- Journal of Clinical Oncology (ASCO), 2020. Hepatitis B Virus Screening and Management for Patients With Cancer Prior to Therapy: ASCO Provisional Clinical Opinion Update
- American Red Cross. Blood Types Explained – A, B, AB and O