Immunology · Test your knowledge series 9
The basic immunology that keeps turning up in pathology exams, in four tables: innate versus adaptive immunity, antibody classes, hypersensitivity reactions and the CD markers you must know. Then take the quiz.
Innate and adaptive immunity
Two arms of the immune system
| Innate | Adaptive | |
|---|---|---|
| Speed | Minutes to hours | Days |
| Specificity | Pattern recognition (Toll-like receptors bind PAMPs and DAMPs) | Highly specific antigen receptors made by gene rearrangement |
| Memory | None (limited trained immunity) | Yes |
| Cells | Neutrophils, macrophages, dendritic cells, NK cells, mast cells | T and B lymphocytes |
| Soluble parts | Complement, acute-phase proteins, cytokines | Antibodies |
Immunoglobulin classes
The five antibody classes
| Class | Structure | Key facts |
|---|---|---|
| IgG | Monomer | Most abundant in serum; crosses the placenta; opsonises; four subclasses |
| IgM | Pentamer (J chain) | First antibody in a primary response; best at fixing complement; does not cross the placenta |
| IgA | Monomer in serum, dimer in secretions | Mucosal defence; in breast milk; IgA deficiency is the commonest primary immunodeficiency |
| IgE | Monomer | Binds mast cells and basophils; allergy and helminth defence |
| IgD | Monomer | On naive B cells with IgM; function in serum unclear |
Hypersensitivity reactions
Gell and Coombs classification
| Type | Mechanism | Examples |
|---|---|---|
| I (immediate) | IgE on mast cells; histamine release within minutes | Anaphylaxis, asthma, hay fever |
| II (antibody-mediated) | IgG or IgM against cell or tissue antigens | Haemolytic transfusion reactions, autoimmune haemolytic anaemia, Goodpasture syndrome, Graves disease, myasthenia gravis |
| III (immune complex) | Antigen–antibody complexes deposit and activate complement | Systemic lupus erythematosus, post-streptococcal glomerulonephritis, serum sickness, Arthus reaction |
| IV (delayed, T cell) | Sensitised T cells and macrophages, 48–72 hours | Tuberculin test, contact dermatitis, granulomas, transplant rejection |
Key CD markers
Markers worth knowing
| Marker | Cells |
|---|---|
| CD3 | All T cells |
| CD4 / CD8 | Helper / cytotoxic T cells |
| CD19, CD20, CD79a | B cells |
| CD16, CD56 | NK cells |
| CD34 | Haematopoietic stem cells and blasts |
| CD45 | All leucocytes (leucocyte common antigen) |
| CD138, CD38 | Plasma cells |
Pathology MCQPractice questions
PRACTICE · SBA1 of 3
Which immunoglobulin crosses the placenta?
Show answer
Correct: C. IgG is actively transported across the placenta through the neonatal Fc receptor; this is why maternal anti-D causes haemolytic disease of the newborn.
PRACTICE · SBA2 of 3
A positive Mantoux (tuberculin) test is which type of hypersensitivity?
Show answer
Correct: D. Sensitised T cells recruit macrophages, producing induration at 48–72 hours: delayed-type hypersensitivity.
PRACTICE · SBA3 of 3
An ABO-incompatible transfusion reaction is an example of:
Show answer
Correct: B. Preformed IgM anti-A or anti-B binds donor red cells and activates complement: antibody-mediated (type II) injury.
Test your knowledge
Key takeaways
- Innate immunity is fast and pattern-based; adaptive immunity is specific and has memory.
- IgG crosses the placenta; IgM is the first responder and best complement fixer; IgA guards mucosa.
- Hypersensitivity: I IgE, II antibody to cells, III immune complexes, IV T cells.
References
Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology, 10th ed. Elsevier; 2022 · Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease, 10th ed. Elsevier; 2020
CourseHematopathology courseRead nextImmunohaematology: ABO, Rh and transfusion reactionsFree practiceGeneral pathology MCQs with virtual slides
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