Why does our body reject transplanted organs?

Answer: Body attacks foreign tissue

New organs have different blood — Wrong. Blood type matching is indeed important and checked before transplant, but even with matched blood types, rejection can occur. The main issue is tissue compatibility (HLA matching). Every person has unique surface proteins (HLA markers) on cells. The immune system recognizes non-self HLA markers and attacks transplanted tissue. Immunosuppressants are needed even with good matches.

Body senses foreign objects — Wrong. The body doesn't just 'sense foreign objects'—the immune system specifically recognizes molecular markers (HLA proteins) on cells. Every person has unique HLA patterns (like a cellular ID). Transplanted organs have different HLA markers, so the immune system identifies them as 'non-self' and attacks. This is why tissue matching (finding similar HLA) and immunosuppressants are important for transplant success.

Body attacks foreign tissue ✓ — Correct! Every person has unique HLA (human leukocyte antigen) proteins on cell surfaces—like a cellular fingerprint. The immune system evolved to attack cells with non-self HLA markers (fighting infections, cancer). Transplanted organs have the donor's HLA markers, triggering immune attack. Even with best HLA matching and blood type compatibility, patients need lifelong immunosuppressants to prevent rejection by preventing this immune recognition and response.