Spotlight #51 – Immunology, Magnetism & Electrochemistry

Three Categories, Three Simulations Each

mysimulator.uk tracks category depth as one quality metric: categories with only one or two simulations offer less exploratory range than those with five or more. Wave 50 targets three such categories.

Immunology: The Complement System

The complement system is among the oldest components of the vertebrate immune system, predating adaptive immunity by hundreds of millions of years. Its name reflects its original description as a heat-labile serum factor that “complemented” the bactericidal activity of antibodies. Today we know it as a tightly regulated proteolytic cascade of ~30 plasma proteins that can be activated by three distinct routes.

What the Complement Cascade Does

• Opsonization — C3b fragments coat pathogen surfaces, tagging them for phagocytosis by macrophages and neutrophils bearing CR1 (complement receptor 1).
• Inflammation — C3a and C5a are potent anaphylatoxins that trigger mast-cell degranulation, smooth-muscle contraction, and neutrophil chemotaxis.
• Direct lysis — The Membrane Attack Complex (C5b-9) forms a 10 nm transmembrane pore, disrupting the osmotic balance of gram-negative bacteria and nucleated cells.

The cascade is normally suppressed on host cells by surface-bound regulators including DAF (CD55), which accelerates C3-convertase decay, and CD59, which blocks MAC assembly. Pathogens lack these regulators, creating the “non-self” discrimination that makes the system safe for the host.

Magnetism: Larmor Precession and MRI

Nuclear magnetic resonance relies on a quantum property — intrinsic spin angular momentum — that every undergraduate physics course covers but few actually visualise well. The Bloch sphere is the standard representation, but textbook drawings are static. Seeing the magnetisation vector actually precess, de-phase, and recover makes the T₁/T₂ distinction intuitive rather than abstract.

Why Different Nuclei Have Different Frequencies

The gyromagnetic ratio γ is a fundamental nuclear constant. It reflects the ratio of the nuclear magnetic moment to the spin angular momentum and varies because different nuclei have different proton/neutron configurations. Protons (¹H) are exceptional: their unpaired proton gives one of the highest γ values among stable nuclei, which is why clinical MRI almost exclusively images water protons despite the body containing many NMR-active nuclei.

³¹P NMR is particularly useful in biochemistry and muscle physiology: it can distinguish ATP, ADP, and inorganic phosphate by their distinct chemical shifts, providing non-invasive windows into cellular energy metabolism.

T₁ vs T₂: Why Two Relaxation Times?

T₁ and T₂ describe fundamentally different physical processes:

• T₁ (spin-lattice) — energy transfer from the spin system to the surrounding lattice (molecular tumbling, phonons). M_z recovers toward thermal equilibrium.
• T₂ (spin-spin) — loss of phase coherence among spins due to local dipolar fields and exchange interactions. M_xy de-phases; energy is redistributed within the spin bath, not to the lattice. Always T₂ ≤ T₁.

MRI sequence design exploits this: T₁-weighted images (short TR/TE) highlight fat and protein-rich tissues; T₂-weighted images (long TR/TE) highlight fluid-rich tissues and oedema.

Electrochemistry: Galvanic Cells and Nernst Thermodynamics

Alessandro Volta’s 1800 “pile” was the first sustained source of electrical current, built by alternating zinc and silver discs separated by brine-soaked cloth. John Frederic Daniell’s 1836 improvement — two electrolyte solutions separated by a porous pot — became the workhorse of 19th-century telegraphy. The Nernst equation, derived in 1889, gave the thermodynamic framework that explains why cell voltage depends on concentration.

Understanding the Nernst Equation Intuitively

The Nernst equation E = E° − (RT/nF) ln Q can be understood from Le Châtelier’s principle: if you increase the concentration of products (anode ions, Zn²⁺), the reaction has less driving force, so E falls. Conversely, a very dilute cathode solution ([Cu²⁺] → 0) makes Q very large and E correspondingly smaller — the cell “prefers” to deposit copper when it is scarce.

Practical Electrochemical Series

The five metal pairs in the simulation span a wide range of the electrochemical series:

• Mg/Cu — 2.71 V: magnesium is extremely reactive; used in seawater-activated batteries and as a sacrificial anode for steel corrosion protection.
• Zn/Ag — 1.56 V: silver-zinc cells power hearing aids and early spacecraft (Apollo program).
• Zn/Cu (Daniell) — 1.10 V: the classic teaching example; identical to the voltage of many household batteries by design.
• Fe/Cu — 0.78 V: relevant to corrosion of iron structures in contact with copper fittings.
• Pb/Cu — 0.47 V: lead-acid derivative; low voltage but high current density achievable.