Immunity — A/L Biology

What must I remember about Immunity?

• Immunity is the body's defense against infectious pathogens and foreign substances, categorized into non-specific Innate Immunity and pathogen-specific Adaptive (Acquired) Immunity.
• Innate Immunity components: Physical/chemical barriers (intact skin, mucosal secretions, stomach HCl, lysozyme in tears/saliva), Phagocytic leukocytes (neutrophils, macrophages, dendritic cells), Natural Killer (NK) cells inducing apoptosis, and antimicrobial proteins (Complement cascade, Interferons).
• The Inflammatory Response: Tissue injury prompts mast cells to release Histamine and macrophages to secrete Cytokines, causing local vasodilation, increased capillary permeability, and chemotaxis of neutrophils to phagocytose pathogens and repair damaged tissue.
• Adaptive Immunity — Humoral (Antibody-Mediated) Immunity: B lymphocytes mature in bone marrow; upon antigen presentation and helper T cell cytokines, B cells differentiate into Plasma cells (secreting specific antibodies: IgG, IgA, IgM, IgE, IgD) and long-lived Memory B cells.
• Adaptive Immunity — Cell-Mediated Immunity: T lymphocytes mature in thymus; Helper T cells (CD4+) recognize antigens on MHC-II and coordinate immune defenses; Cytotoxic T cells (CD8+) recognize infected cells via MHC-I, releasing Perforin and Granzymes to trigger apoptosis.

Test yourself (1 minute)

1. Primary vs Secondary Immune Response

   Primary response has a lag period (5-10 days) with low IgM-dominated titer; secondary exposure triggers rapid, massive, high-affinity IgG production driven by memory cells.

2. Active vs Passive Immunity

   Active immunity involves production of one's own antibodies/memory cells (natural: infection; artificial: vaccines); Passive immunity provides immediate, temporary antibodies without memory (natural: maternal IgG/IgA via placenta/colostrum; artificial: anti-venom).

3. Major Histocompatibility Complex (MHC)

   MHC-I molecules present endogenous antigens to CD8+ Cytotoxic T cells on all nucleated cells; MHC-II molecules present exogenous antigens to CD4+ Helper T cells exclusively on APCs (dendritic cells, macrophages, B cells).

Practise Immunity questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 5

What should I study first?

What comes next?

Next in the syllabus

Where does Immunity fit in the syllabus?

Year 2 · Term 1