Explain how antigens and antibodies work and how labs use them in blood typing and immunoassays.
Why it matters
The immune system is the reason many lab tests work. Pregnancy tests, COVID-19 rapid tests, blood typing, allergy tests and ELISA assays all use antibodies that bind specific targets. Biochemists engineer antibodies as drugs; medical laboratory technologists use them every day in blood banks and immunology sections. To understand those tests, you need to know what antigens and antibodies are.
Two lines of defence
Innate immunity is fast (minutes to hours) and general. It includes:
- barriers: skin, mucus, stomach acid
- phagocytes such as neutrophils and macrophages that engulf microbes (phagocytosis, from module 1)
- inflammation: redness, heat, swelling, which brings more cells to the site
- fever and complement proteins that punch holes in bacteria
Adaptive immunity is slower the first time (days) but specific, and it remembers. It relies on lymphocytes:
- B cells make antibodies.
- T cells: helper T cells coordinate the response; cytotoxic T cells kill infected or abnormal cells.
Antigens and antibodies
An antigen is any molecule the immune system can recognise, usually a protein or sugar on a microbe, a cell or a toxin. The small region an antibody actually binds is the epitope.
An antibody (immunoglobulin, Ig) is a Y-shaped protein made by B cells (as plasma cells). It has:
- two identical arms with variable regions at the tips that bind one specific epitope, like a lock and key
- a stem (constant region) that signals to other immune cells
Antibodies neutralise toxins and viruses, clump (agglutinate) microbes together and tag them for phagocytes.
The main classes:
| Class | Notes |
|---|---|
| IgM | first antibody made in a new infection; large five-unit form |
| IgG | most common in blood; made later and in memory responses; crosses the placenta |
| IgA | in mucus, saliva, tears and breast milk |
| IgE | involved in allergies and parasites |
| IgD | on the surface of B cells |
A positive IgM suggests a recent infection; IgG alone often suggests past infection or vaccination. Clinicians interpret these patterns; the lab must report them accurately.
Memory and vaccines
After an infection, some B and T cells become long-lived memory cells. The next time the same antigen appears, the secondary response is faster, larger and mostly IgG. Vaccines use this: they present a harmless form of an antigen (an inactivated virus, a protein subunit or mRNA that instructs your cells to make a protein) so memory forms without the disease.
Worked example 1: ABO blood typing
Red blood cells carry antigens A, B, both or neither. Plasma contains antibodies against the antigens a person does not have.
| Blood group | Antigens on red cells | Antibodies in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |
In forward typing, the lab mixes a patient's red cells with anti-A and anti-B reagents. Agglutination (clumping) with anti-A only means group A. Clumping with both means AB; with neither means O. This is why a transfusion of the wrong group is dangerous: the recipient's antibodies attack the donor's cells. Blood bank work is highly regulated and always done under trained supervision with confirmatory checks.
Worked example 2: a lateral flow test
A home rapid antigen test is an immunoassay on a strip:
- The sample flows over labelled antibodies (attached to coloured particles) that bind the viral antigen.
- At the test line, fixed antibodies capture the antigen-antibody complex, and the colour builds up into a visible line.
- At the control line, other fixed antibodies capture the labelled antibodies whether or not antigen is present. No control line means the test is invalid.
Laboratory ELISA tests use the same idea in plastic wells, with an enzyme that produces a colour change measured by a plate reader, which gives a number rather than a yes/no line.
Common mistakes
- Mixing up antigen and antibody. The antigen is the target; the antibody is the protein that binds it.
- Thinking group O has no antibodies. Group O has no A or B antigens but has both anti-A and anti-B.
- Ignoring the control line on a lateral flow test.
- Assuming the immune system only fights microbes. It also causes allergies, transplant rejection and autoimmune disease.
Quick recap
- Innate immunity is fast and general; adaptive immunity is specific and has memory.
- B cells make antibodies; T cells help or kill.
- Antibodies bind specific epitopes on antigens and neutralise, clump or tag them.
- IgM comes first; IgG is the main long-term and memory antibody.
- Blood typing, lateral flow tests and ELISA all use antigen-antibody binding.