AIM • Step 10
3rd Year MBBS • Blood and Immunology
Student Memory Support
Foundations of Immunity, Immune Responses, Antibodies and Complement
High-yield memory reinforcement for rapid revision of immune cells, adaptive responses, immunoglobulins and complement.
1. High-Yield Flashcards
Tap each question to reveal the answer.
Q: What is the main difference between innate and adaptive immunity?
A: Innate immunity is rapid and recognizes shared microbial patterns; adaptive immunity is antigen-specific and develops memory.
Q: What forms the first line of immune defense?
A: Skin, mucosal epithelium and antimicrobial substances in secretions.
Q: Which immune cell is particularly effective at activating naïve T lymphocytes?
A: Dendritic cell.
Q: What is the main role of plasma cells?
A: Secretion of large quantities of antibodies.
Q: Which cells provide natural cytotoxic activity without antigen-specific T-cell receptors?
A: Natural killer cells.
Q: How do B and T lymphocytes differ in antigen recognition?
A: B cells can recognize native antigen; T cells generally recognize peptide presented by MHC molecules.
Q: What is clonal selection?
A: An antigen selects and activates lymphocyte clones whose receptors already recognize that antigen.
Q: What is the main function of helper T lymphocytes in humoral immunity?
A: They provide signals that promote B-cell proliferation, differentiation, antibody production, class switching and memory.
Q: Which MHC classes are mainly recognized by CD4+ and CD8+ T cells?
A: CD4+ T cells recognize MHC class II; CD8+ T cells recognize MHC class I.
Q: Which cytotoxic molecules are released by CD8+ T lymphocytes?
A: Perforin and granzymes.
Q: What is the basic structure of an immunoglobulin molecule?
A: Two identical heavy chains and two identical light chains with variable and constant regions.
Q: What structural features characterize secretory IgA?
A: Two IgA monomers linked by a J chain and associated with a secretory component.
Q: What changes during immunoglobulin class switching?
A: The heavy-chain constant region changes, altering effector function while antigen specificity is retained.
Q: Which three pathways activate complement?
A: Classical, lectin and alternative pathways.
Q: What are the key functions of C3b, C5a and C5b-C9?
A: C3b opsonizes, C5a recruits and activates leukocytes, and C5b-C9 forms the membrane attack complex.
Q: Which complement deficiencies are linked with recurrent pyogenic and recurrent Neisseria infections?
A: C3 deficiency predisposes to recurrent pyogenic infection; C5-C9 deficiency predisposes to recurrent Neisseria infection.
2. Mnemonics
Mnemonic Title: Immunoglobulin Classes
GAMED
Meaning: IgG • IgA • IgM • IgE • IgD
Mnemonic Title: Complement Activation Pathways
CLA
Meaning: Classical • Lectin • Alternative
Mnemonic Title: Key Complement Effects
OIL
Meaning: Opsonization • Inflammation • Lysis
3. Memory Tables
Innate vs Adaptive Immunity
| Feature | Innate | Adaptive |
|---|---|---|
| Speed | Rapid | Slower on first exposure |
| Recognition | Shared microbial patterns | Specific antigens |
| Main cells | Phagocytes, dendritic cells, NK cells | B and T lymphocytes |
| Memory | No classical antigen-specific memory | Present |
Major Immunoglobulin Classes
| Class | High-Yield Feature | Major Link |
|---|---|---|
| IgG | Major circulating antibody | Placental transfer, opsonization, neutralization |
| IgA | Secretory form is mainly dimeric | Mucosal and breast-milk protection |
| IgM | Pentamer when secreted | Early response, classical complement activation |
| IgE | Binds mast cells and basophils | Immediate hypersensitivity |
| IgD | Membrane immunoglobulin | B-cell antigen receptor |
4. Rapid Revision Points — Last-Minute Revision
Must Remember:
- Innate immunity acts rapidly; adaptive immunity provides high specificity and memory.
- Dendritic cells are key antigen-presenting cells for activation of naïve T lymphocytes.
- B-cell activation produces plasma cells for antibody secretion and memory B cells for future responses.
- CD4+ T cells mainly recognize peptide-MHC II; CD8+ T cells mainly recognize peptide-MHC I.
- Humoral immunity is most effective against extracellular microbes and toxins.
- Cell-mediated immunity is particularly important against intracellular microbes and infected cells.
- Class switching changes antibody effector function but retains antigen specificity.
- Maternal IgG protects the fetus; breast-milk secretory IgA protects infant mucosal surfaces.
- All three complement pathways converge on activation of C3.
- C3b promotes phagocytosis, C5a recruits leukocytes and C5b-C9 produces membrane attack complex formation.
Common KMU confusion: Antibodies are adaptive immune molecules, whereas complement can function in innate immunity and can also be activated by antibody through the classical pathway.
5. Clinical Memory Hooks
Recurrent pyogenic bacterial infections
→ poor complement-mediated opsonization → think C3 deficiency.
→ poor complement-mediated opsonization → think C3 deficiency.
Recurrent Neisseria infections
→ defective membrane attack complex → consider C5-C9 deficiency.
→ defective membrane attack complex → consider C5-C9 deficiency.
Virus replicating inside host cells
→ intracellular peptide on MHC I → CD8+ cytotoxic T-cell response.
→ intracellular peptide on MHC I → CD8+ cytotoxic T-cell response.
Breast-fed infant receiving mucosal protection
→ secretory IgA in breast milk → neutralization at mucosal surfaces.
→ secretory IgA in breast milk → neutralization at mucosal surfaces.
6. Starred High-Yield Exam Points
- ⭐ CD4+ T lymphocytes mainly recognize peptide on MHC class II; CD8+ T lymphocytes mainly recognize peptide on MHC class I.
- ⭐ Class switching alters the heavy-chain constant region and effector function without changing antigen specificity.
- ⭐ IgG crosses the placenta; secretory IgA provides mucosal protection to the breast-fed infant.
- ⭐ C3b is a major complement opsonin, while C5a is an important leukocyte chemotactic and activating factor.
- ⭐ C5b-C9 forms the membrane attack complex; deficiency predisposes particularly to recurrent Neisseria infection.
- ⭐ The secondary immune response is faster and stronger because memory lymphocytes were generated during the primary response.