Q.Transplantation of tissues/organs to save certain patients often fails due to rejection of such tissues/organs by the patient. Which type of immune response is responsible for such rejections?

When a foreign tissue or organ is transplanted into a patient's body, the recipient's immune system recognizes the graft cells as "non-self" because they carry different surface markers (MHC molecules). The body mounts a defense to eliminate these foreign cells, leading to graft rejection.

This rejection is primarily driven by cell-mediated immunity, where T-lymphocytes play the central role. Specifically, cytotoxic T cells recognize the foreign MHC antigens on the transplanted tissue and directly attack and destroy the graft cells. Helper T cells also participate by coordinating the immune response. This is distinct from humoral immunity, which involves antibodies produced by B cells and is more effective against pathogens in body fluids rather than transplanted solid tissues. …

When a surgeon transplants a kidney, heart, or any organ from one person (the donor) into another (the recipient), the recipient's immune system faces a profound challenge: it encounters cells bearing surface markers—specifically, proteins called Major Histocompatibility Complex (MHC) molecules or Human Leukocyte Antigens (HLA)—that differ from its own. The immune system evolved to distinguish "self" from "non-self," and a transplanted organ, unless from an identical twin, carries non-self markers. This triggers an immune response aimed at destroying what the body perceives as an invader.

The rejection process is primarily orchestrated by T-lymphocytes, the soldiers of cell-mediated immunity. Unlike humoral immunity, which relies on antibodies secreted by B-cells circulating in blood and lymph to neutralize pathogens, cell-mediated immunity involves direct cellular attack. Here's how it unfolds in transplant rejection:

• Recognition: Recipient T-cells encounter the foreign MHC molecules on the donor organ's cells. These T-cells have receptors that can distinguish the donor's tissue as "non-self."
• Activation and proliferation: Once activated, these T-cells multiply rapidly, creating an army specifically targeted against the graft.
• Attack: Cytotoxic T-lymphocytes (a subset of T-cells) migrate to the transplanted organ and directly kill its cells by releasing toxic substances. Helper T-cells amplify the response by recruiting macrophages and other immune cells, leading to inflammation and tissue damage.

This is why transplant patients must take immunosuppressive drugs—medications that dampen T-cell activity—to prevent their own immune system from destroying the life-saving organ. …

Define each option as a distinct immunological term FIRST, then test the scenario against each definition: auto-immune = attacking self; humoral = antibody-based; 'physiological' = not a real immunology category; cell-mediated = direct T-cell attack on foreign MHC-bearing cells. A transplant is foreign (not self) tissue destroyed by direct ce …