Unit 1: Immunity, antigens and antibodies
Immunology and Mycology notes · PTU syllabus (BMLS407-18)
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Unit summary
Immunology studies how the body defends itself. This unit covers the history of immunology, innate and acquired immunity, types of antigens and determinants of antigenicity, and the structure and properties of immunoglobulins.
After this unit you can
- Trace the history of immunology
- Distinguish innate and acquired immunity
- Describe antigens and factors affecting antigenicity
- Describe the structure and classes of immunoglobulins
PTU syllabus topics
- History and introduction to immunology
- innate and acquired immunity
- types of antigens and determinants of antigenicity
- structure and properties of immunoglobulins
Present
From birth
Develops after exposure
Specificity
Non-specific
Specific to an antigen
Memory
None
Yes: faster second response
Examples
Skin, phagocytes, complement
Antibodies (B cells), T cells
Topic 1
History of immunology
- 1
Edward Jenner (1796)
Cowpox vaccination against smallpox
- 2
Louis Pasteur (1880s)
Attenuated vaccines (chicken cholera, anthrax, rabies)
- 3
Metchnikoff (1882)
Phagocytosis — cellular immunity
- 4
von Behring and Kitasato (1890)
Antitoxins — humoral immunity
- 5
Landsteiner (1900)
ABO blood groups
- 6
Burnet (1957)
Clonal selection theory
- 7
Köhler and Milstein (1975)
Monoclonal antibodies
Topic 2
Innate and acquired immunity
Presence
From birth; non-specific
Develops after exposure; specific
Memory
None
Yes — faster secondary response
Components
Skin, mucous membranes, lysozyme, phagocytes, NK cells, complement, fever, inflammation
B cells (antibodies), T cells (cell-mediated)
Active natural
After infection
Active artificial
After vaccination
Passive natural
Maternal IgG across the placenta, IgA in breast milk
Passive artificial
Ready-made antibodies — antitoxins, immunoglobulin (rabies, tetanus)
Topic 3
Antigens and determinants of antigenicity
- Antigen: substance that reacts with antibodies or T cells; immunogen: antigen that induces an immune response. Epitope (antigenic determinant): the small part recognised by an antibody. Hapten: small molecule antigenic only when attached to a carrier protein (e.g., penicillin).
- Foreignness
- More foreign, more immunogenic
- Molecular size
- Above about 10,000 Da is better
- Chemical complexity
- Proteins best; polysaccharides moderate; lipids poor
- Stability and degradability
- Must be processed
- Route and dose
- Parenteral better; optimal dose
- Adjuvants
- Alum enhances responses
- Host factors
- Age, genetics (HLA), nutrition
- Types: exogenous, endogenous, autoantigens, alloantigens (blood groups), heterophile antigens (shared across species — Weil–Felix, Paul–Bunnell), superantigens (staphylococcal toxins).
Topic 4
Structure and properties of immunoglobulins
- Basic structure: Y-shaped; two identical heavy and two light (κ or λ) chains joined by disulphide bonds. Fab fragments (variable regions) bind antigen; Fc fragment (constant region) fixes complement and binds cells. Papain splits into 2 Fab + Fc.
IgG
Monomer; 75–80% of serum Ig; crosses placenta
Secondary response; neutralises toxins; opsonisation
IgM
Pentamer; largest; first made
Primary response; best agglutinator and complement activator; marker of recent infection
IgA
Dimer in secretions (secretory component)
Mucosal immunity — tears, saliva, milk
IgE
Monomer; binds mast cells
Allergy and parasites
IgD
Monomer; on B-cell surface
B-cell activation
Key terms
- Innate immunity
- Non-specific defence present from birth
- Epitope
- Part of an antigen recognised by an antibody
- Hapten
- Small molecule antigenic only with a carrier
- Fab
- Antigen-binding fragment of an antibody
- IgM
- First antibody class produced in infection
Quick revision
- Jenner, Pasteur, Metchnikoff, Behring, Landsteiner, Burnet, Köhler and Milstein.
- Innate vs acquired; active, passive, natural, artificial.
- Antigen, immunogen, epitope, hapten; factors affecting antigenicity; types.
- Ig structure; IgG, IgM, IgA, IgE, IgD.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Distinguish active and passive immunity.
- Q2.What is a hapten?
- Q3.Name three factors affecting antigenicity.
- Q4.Which immunoglobulin crosses the placenta?
- Q5.Which class indicates recent infection?
- Q6.What is the role of IgE?
Long-answer questions
- Q1.Compare innate and acquired immunity.
- Q2.Describe antigens and determinants of antigenicity.
- Q3.Describe the structure and classes of immunoglobulins.
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