Graft Rejection: Why Transplanted Organs Are Rejected (Cell-Mediated Immunity)
Biology · Human Health and Diseases · NEET
Graft rejection happens because the body's cell-mediated immune response (carried out by T-lymphocytes) treats the transplanted organ as "nonself" and attacks it. This is why tissue matching and blood group matching are done before a transplant, and why the patient must take immunosuppressant drugs for life. Memory hook: "Graft rejection = T-cell job = Cell-mediated."
Graft rejection flow: the donor organ is nonself, so T-lymphocytes attack it. This is the cell-mediated immune response, reduced by matching and lifelong immunosuppressants.
Your doubts, answered
Which immune response is responsible for graft rejection?
The cell-mediated immune response is responsible for graft rejection. It is carried out by T-lymphocytes, not by antibodies. NCERT states this directly, and NEET has asked it in 2017 and 2019, so remember it as a fixed fact.
Is graft rejection humoral or cell-mediated?
It is cell-mediated. The humoral (antibody-mediated) response acts against free-floating antigens in body fluids, but a transplanted organ is a solid mass of nonself cells, so the T-lymphocytes of cell-mediated immunity attack it. If you mark 'humoral', the answer is wrong for NEET.
Why does the body reject a transplanted organ?
The body can tell 'self' from 'nonself'. The donor organ carries different surface proteins, so it is seen as nonself (foreign). The cell-mediated immune response then attacks and rejects it. This is why a graft from just any source (animal, another human) cannot be used freely.
What is the difference between graft rejection and autoimmune disease?
In graft rejection the body attacks NONSELF (foreign transplanted tissue) using cell-mediated immunity. In an autoimmune disease the body wrongly attacks its OWN self-cells, for example rheumatoid arthritis. NEET 2016 used this exact distinction, so do not mix them up.
Why do transplant patients take immunosuppressants for life?
Even after tissue and blood group matching, the graft is never a perfect match, so the T-lymphocytes would still slowly reject it. Immunosuppressant drugs lower this immune attack so the organ survives. The patient must take them all their life.
Why is tissue matching and blood group matching done before a transplant?
Matching makes the donor organ as similar as possible to the recipient, so the cell-mediated response sees it as less foreign. This reduces the chance and speed of rejection. NCERT says both tissue matching and blood group matching are essential before any graft.
⚠️ The NEET trap ✗ Choosing 'Humoral immune response' because you think antibodies fight anything foreign. ✓ The correct answer is the cell-mediated immune response, done by T-lymphocytes. 🧠 Antibodies (humoral) chase free antigens; a whole donor organ is a block of nonself cells, so T-cells (cell-mediated) do the rejecting.
Real NEET questions
2017
Transplantation of tissues/organs fails often due to non-acceptance by the patient's body. Which type of immune-response is responsible for such rejections?
A · Autoimmune response
B · Cell-mediated immune response ✓
C · Hormonal immune response
D · Physiological immune response
Solution: Cell-mediated immune response. The body differentiates 'self' from 'nonself', and T-lymphocytes carry out the cell-mediated response that rejects the graft. Autoimmune response attacks one's own cells; there is no 'hormonal' immune response, and physiological barriers belong to innate immunity. NCERT Ch7: 'the cell-mediated immune response is responsible for the graft rejection.'
2019
Which of the following immune responses is responsible for rejection of kidney graft?
A · Auto-immune response
B · Humoral immune response
C · Inflammatory immune response
D · Cell-mediated immune response ✓
Solution: Cell-mediated immune response. A kidney graft is nonself tissue, and T-lymphocytes reject it. Humoral (antibody) response acts on free antigens, not a solid organ; auto-immune and inflammatory responses are unrelated to graft rejection. This is why tissue/blood group matching and lifelong immunosuppressants are needed.
2016
In higher vertebrates, the immune system can distinguish self-cells and non-self. If this property is lost due to genetic abnormality and it attacks self-cells, then it leads to:
A · Allergic response
B · Graft rejection
C · Auto-immune disease ✓
D · Active immunity
Solution: Auto-immune disease. When the body attacks its OWN self-cells (e.g. rheumatoid arthritis), it is autoimmunity. Note the trap: graft rejection is the attack on NONSELF transplanted tissue (cell-mediated), not on self-cells, so 'Graft rejection' is wrong here.
Solved Human Health and Diseases NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
T-lymphocytes. They carry out the cell-mediated immune response that attacks the nonself transplanted organ.
Can a graft be taken from any animal or person?
No. NCERT says a graft from just any source (an animal, another primate, or any human) would be rejected sooner or later, because the body sees it as nonself.
Which drug is used to prevent graft rejection?
Immunosuppressants. NEET also links this to Cyclosporin A, an immunosuppressive drug produced by the fungus Trichoderma polysporum (from the Microbes chapter).
Is graft rejection a specific or non-specific response?
It is a specific (acquired) response, part of cell-mediated immunity. Innate immunity is the non-specific defence present at birth, which is not what rejects grafts.
Does blood group matching alone prevent rejection?
No. Both tissue matching and blood group matching are needed, and even then the patient still takes immunosuppressants for life, because the match is never perfect.
