Immunity: MDCAT Notes and MCQs
Defence overview
- Non-specific: skin, mucus, lysozyme, stomach acid, phagocytes (neutrophils, macrophages), inflammation, fever, interferons.
- Specific (immune response): targets particular antigens; has specificity, diversity and memory. Carried out by lymphocytes.
Lymphocytes
| B-lymphocytes | T-lymphocytes |
|---|---|
| Formed and mature in bone marrow | Formed in bone marrow, mature in thymus |
| Humoral (antibody-mediated) immunity | Cell-mediated immunity |
| Activated B-cells → plasma cells (secrete antibodies) + memory B-cells | Cytotoxic/killer T (destroy infected & cancer cells), helper T (activate B & T cells), suppressor T, memory T |
| Act against bacteria & viruses in body fluids | Act against infected cells, fungi, transplants (graft rejection), cancer |
Antibodies (immunoglobulins)
- Y-shaped proteins: 2 heavy + 2 light chains joined by disulphide bonds; each has variable region (antigen-binding site) and constant region. Each antibody has 2 binding sites.
- Classes: IgG (most abundant ≈ 75–80%, crosses placenta), IgM (largest, pentamer, first produced in primary response), IgA (in secretions — milk, saliva, tears), IgE (allergy, histamine release), IgD (B-cell receptor).
- Actions: neutralization, agglutination, precipitation, opsonization (enhance phagocytosis), complement activation.
Primary vs secondary response
First exposure → slow, weak primary response; memory cells make the secondary response faster, stronger and longer — the basis of vaccination.
Types of immunity
| Natural | Artificial | |
|---|---|---|
| Active (own antibodies, long-lasting) | Recovering from infection | Vaccination |
| Passive (ready-made antibodies, short-lived) | Antibodies via placenta / colostrum | Antiserum injection (anti-tetanus, anti-rabies, anti-venom) |
Edward Jenner (1796) developed the smallpox vaccine using cowpox. Failure of tolerance to "self" → autoimmune disease.
PMDC learning outcomes for this unit
- Specific Defense Mechanism: 12.1 Define and discuss the functions and importance of specific defense mechanisms.