Immunity, Infection and Forensics

Welcome to Topic 6: Immunity, Infection and Forensics!

In this chapter, we are going to become biological detectives. We will start by looking at how scientists use clues from a body to solve crimes (Forensics), then dive deep into the microscopic world of pathogens like bacteria and viruses, and finally explore how our incredible immune system fights back. Don't worry if it seems like a lot to take in at first—we'll break it down step-by-step!

Part 1: Forensic Biology – The Science of Time and Identity

When a body is found, one of the first questions is: "When did this person die?" Biologists use several clues to determine the Time of Death (ToD).

1.1 Determining Time of Death

There are five main "clocks" that forensic scientists look at:

• Body Temperature: From the moment of death, the body begins to cool. This follows a "sigmoidal" curve. Factors like body size, clothing, and air temperature affect how fast it cools.
• Rigor Mortis (Muscle Contraction): About 3–8 hours after death, muscles become stiff. This happens because ATP is no longer produced, so the muscle fibers can't "unlock." After about 36 hours, the muscles relax again as tissue breaks down.
• Decomposition: Microorganisms (like bacteria and fungi) break down tissues. This causes skin discoloration and "bloating" as gases are released.
• Forensic Entomology: Different insects (like blowflies and beetles) arrive at a body at different times. By looking at which species are present and what stage of their life cycle they are in (eggs, larvae, pupae), scientists can estimate the ToD.
• Succession: Just like plants in a forest, different types of organisms colonize a body in a predictable order as it changes over time.

1.2 The Role of Microorganisms

Microorganisms are the planet's ultimate recyclers. They play a vital role in decomposing organic matter, which releases carbon dioxide back into the atmosphere (the carbon cycle) and returns nutrients to the soil.

1.3 DNA Profiling and PCR

Every person (except identical twins) has unique DNA. DNA profiling is used to identify individuals or find genetic relationships.

PCR (Polymerase Chain Reaction): Often, there isn't enough DNA at a crime scene. PCR is like a "biological photocopier."
1. Denaturation: Heat to \( 95^\circ\text{C} \) to separate DNA strands.
2. Annealing: Cool to \( 55^\circ\text{C} \) so primers can bind.
3. Extension: Heat to \( 72^\circ\text{C} \) so DNA polymerase can build new strands.

Gel Electrophoresis: This is used to separate DNA fragments by length. Small fragments move faster through the gel toward the positive electrode, creating a unique pattern of bands.

Quick Review: Time of death can be found via temperature, rigor mortis, decomposition, bugs (entomology), and succession. PCR "copies" DNA, and gel electrophoresis "sorts" it.

Part 2: The Biological Invaders – Bacteria and Viruses

To understand infection, we need to know what we are up against.

2.1 Bacteria vs. Viruses

Bacteria: These are living, prokaryotic cells. They have a cell wall, a cell membrane, and circular DNA (plasmids). They can reproduce on their own through binary fission.

Viruses: These are not cells. They are much smaller and consist of genetic material (DNA or RNA) inside a protein coat (capsid). They are "hijackers"—they must enter a host cell to reproduce.

2.2 HIV and TB: Two Major Threats

The syllabus requires you to know these two specific infections:

• HIV (Human Immunodeficiency Virus): It targets T-helper cells. By destroying these, the immune system becomes so weak that the person develops AIDS and can die from "opportunistic infections" that a healthy person would easily fight off.
• Mycobacterium tuberculosis (TB): This bacteria usually infects the lungs. It can survive inside macrophages (immune cells) by building a protective wall around itself (a tubercle). If the immune system weakens, the bacteria "wake up" and destroy lung tissue.

2.3 One Gene, Many Proteins

Did you know? Humans have about 20,000 genes but make over 100,000 proteins. How? Through post-transcriptional changes. After mRNA is made, "introns" (non-coding bits) are removed, and "exons" (coding bits) are spliced together in different orders. This is called alternative splicing.

Key Takeaway: Bacteria are independent cells; viruses are genetic hijackers. HIV destroys the immune system's "general" (T-helper cell), and TB hides inside the system's "guards" (macrophages).

Part 3: The Body’s Defense – Immunity

The body is like a fortress. If an invader gets in, there are several levels of defense.

3.1 Barriers to Entry

Before an infection starts, the body tries to keep pathogens out using:
- Skin: A physical barrier.
- Stomach Acid: Kills most bacteria in food.
- Skin and Gut Flora: "Friendly" bacteria that compete with pathogens for space and nutrients.

3.2 Non-Specific Response (The Rapid Response Team)

If pathogens get past the barriers, the body uses "non-specific" defenses (they treat all invaders the same):

• Inflammation: Blood flow increases to the area, bringing white blood cells.
• Lysozyme Action: Enzymes in tears and saliva that destroy bacterial cell walls.
• Interferon: Proteins that help stop viruses from spreading to healthy cells.
• Phagocytosis: White blood cells (macrophages) literally eat and digest the pathogens.

3.3 Specific Immune Response (The Special Forces)

This involves Antigens (markers on pathogens) and Antibodies (proteins that stick to pathogens).

T Cells (made in bone marrow, mature in Thymus):
- T-Helper: Stimulates other cells to fight.
- T-Killer: Destroys infected host cells.
- T-Memory: "Remembers" the pathogen for next time.

B Cells (made and mature in Bone marrow):
- B-Effector (Plasma Cells): Produce antibodies.
- B-Memory: Provide long-term immunity.

3.4 Types of Immunity

Don't worry if this seems tricky; just look at the two words in each term:

• Natural: You got infected or got antibodies from your mother.
• Artificial: You got a vaccination.
• Active: Your body made the antibodies (long-lasting).
• Passive: You were given antibodies (immediate but short-term).

Quick Memory Aid:
B-cells = Bullet-makers (they fire antibodies).
T-cells = Tough-guys (they kill cells directly or lead the army).

Part 4: The Evolutionary Race and Antibiotics

Pathogens and hosts are in an "evolutionary arms race." As we develop better immune systems, pathogens evolve evasion mechanisms to hide or change their antigens.

4.1 Antibiotics

Antibiotics only work on bacteria, not viruses!
- Bactericidal: Kill bacteria (e.g., by destroying the cell wall).
- Bacteriostatic: Stop bacteria from reproducing (e.g., by stopping protein synthesis).

4.2 Hospital Acquired Infections (HAIs)

Hospitals can be breeding grounds for "superbugs" like MRSA. To control this, hospitals use strict codes of practice:
- Doctors/nurses washing hands between patients.
- Isolating infected patients.
- Only prescribing antibiotics when absolutely necessary to prevent antibiotic resistance.

Common Mistake to Avoid: Never say "bacteria become immune to antibiotics." Instead, say they develop resistance through genetic mutations and natural selection.

Summary: We fight pathogens with physical barriers, non-specific "eating" (phagocytosis), and specific "memory" (B and T cells). Antibiotics help us kill bacteria, but we must use them carefully to avoid resistance.