Antigen–Antibody Reactions medical comic
This comic is a serology survival guide. It explains the rules of antigen–antibody binding and the stages of a serological reaction, then how to judge a test with sensitivity, specificity and predictive values, before working through the classic tests — precipitation, agglutination, complement fixation, labelled assays and flow cytometry.
For MBBS, NEET-PG, FMGE, NEXT and USMLE revision.
Antigen–antibody reactions: what the comic covers
High-yield points
- Antigen–antibody binding is specific, non-covalent and reversible; it depends on affinity and avidity.
- Reactions have a primary (binding), secondary (visible: precipitation, agglutination) and tertiary (biological) stage.
- Antibody excess causes the prozone effect and antigen excess the postzone effect — both give false negatives; the zone of equivalence gives the best reaction.
- Sensitivity is the ability to detect true positives; specificity is the ability to exclude true negatives; predictive values depend on how common the disease is.
- Precipitation tests include Oudin single diffusion, Oakley–Fulthorpe, radial immunodiffusion, Ouchterlony double diffusion, immunoelectrophoresis, counter-immunoelectrophoresis and rocket electrophoresis; VDRL is a flocculation test for syphilis.
- Agglutination tests include blood grouping, Widal, Coombs tests and heterophile tests such as Weil–Felix and Paul–Bunnell.
- Complement fixation tests use complement consumption, read through a haemolytic indicator system.
- Labelled assays — ELISA, immunofluorescence and others — and flow cytometry give sensitive detection and cell counting (e.g. CD4 counts).
Mnemonics from the comic
- Immunodiffusion: Ouchterlony checks antigen identity at the door.
Quick questions
What is the prozone phenomenon?
A false-negative result caused by too much antibody: antigen sites are saturated, so lattices do not form. Diluting the serum restores the reaction.
What is the difference between sensitivity and specificity?
Sensitivity is the proportion of people with the disease who test positive. Specificity is the proportion of people without the disease who test negative. Screening favours high sensitivity; confirmation favours high specificity.
What does the Coombs test detect?
The direct Coombs test finds antibody or complement already on red cells, as in autoimmune haemolysis or haemolytic disease of the newborn. The indirect test finds antibodies against red cells in serum, as in antenatal and pre-transfusion screening.
Is this comic free to read?
The first pages are free to preview. Members read the full comic, and you can sign up free to read one comic in full.
Read next
- Antibodies medical comic
- Antigens medical comic
- Complement System medical comic
- Immune Response medical comic
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