Question 1

Why does the body reject transplanted organs?

Accepted Answer

Because the immune system is designed to identify and destroy **anything not recognised as self**, and a donated organ is genetically foreign tissue.

**How the immune system tells self from non-self.** Nearly every cell displays surface proteins called **HLA (human leukocyte antigen)** molecules — the human version of the major histocompatibility complex. These act as an identity badge, and they are extraordinarily variable between individuals. Your immune system learns your own HLA pattern during development and treats anything different as foreign.

A transplanted organ carries the **donor's** HLA. T cells recognise it as non-self and attack.

**The types of rejection:**

**Hyperacute** — within minutes to hours, caused by pre-existing antibodies, usually from blood group incompatibility. The organ fails immediately. Crossmatching before surgery exists to prevent it, and it is now rare.

**Acute** — days to months, T-cell mediated. Common, often treatable if caught early with increased immunosuppression, which is why post-transplant monitoring is intensive.

**Chronic** — months to years. Gradual fibrosis and vascular narrowing. Poorly understood, difficult to treat, and the main reason transplanted organs have finite lifespans.

**How rejection is managed:**

**Tissue matching.** The closer the HLA match, the lower the risk — which is why related donors are often preferable and why matching registries exist.

**Lifelong immunosuppression**, the central trade-off in transplantation. Suppressing the immune system prevents rejection and simultaneously increases susceptibility to **infection and certain cancers**. Getting the balance right is the ongoing clinical challenge, and the drugs carry substantial side effects.

**Why some tissues are different.** The **cornea** can often be transplanted without matching or systemic immunosuppression, because it has no blood supply — an **immune-privileged** site the immune system does not patrol normally.

**Graft-versus-host disease** is the reverse problem, occurring in bone marrow transplants where **donor** immune cells attack the recipient.

Question 2

How does organ donation actually work?

Accepted Answer

Through a tightly time-limited process that depends on how someone dies as much as on whether they consented — which is why donation is possible far less often than people assume. The constraint

Question 3

What actually happens when you give blood?

Accepted Answer

A screening process, a donation taking around ten minutes, and then a considerable amount of laboratory work — most of which donors never see and which explains why the appointment takes an hour for

Question 4

What actually happens in your body when you get a vaccine?

Accepted Answer

A vaccine gives your immune system a rehearsal — exposure to something resembling a pathogen, so that memory is built without the illness. The two-stage immune response. Meeting a new pathogen, the

Question 5

What is sepsis, and why is it so dangerous?

Accepted Answer

Sepsis is the body's own response to an infection going wrong and damaging its own tissues and organs. It is not a specific infection — it is what can happen with almost any infection, anywhere in

Question 6

Why does a bruise change colour as it heals?

Accepted Answer

The colour sequence is your body dismantling the haemoglobin that leaked out of broken blood vessels, and each shade corresponds to a different breakdown product. A bruise forms when blunt impact

Question 7

What do the kidneys do, and what is dialysis?

Accepted Answer

The kidneys do far more than make urine. They are continuous chemical regulators, and dialysis replaces only part of what they do — which is why it is a treatment rather than a cure. What healthy

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