What is Cellular Immunity?
Cellular immunity is the branch of adaptive immunity where T lymphocytes directly destroy infected, cancerous, or foreign cells — no antibodies involved. It relies on cell-to-cell recognition rather than circulating proteins. It's essential for clearing viruses, intracellular bacteria, tumor cells, and driving transplant rejection.
Cellular immunity is the antibody-free branch of adaptive immunity in which cytotoxic T cells (CD8+) recognize infected or abnormal cells via MHC class I and kill them directly.
- •Mediated by T lymphocytes
- •Kills infected/abnormal cells directly
- •Uses cytotoxic T cells (CD8+)
- •No antibodies involved
- •Key vs viruses, cancer, transplants
- •Mediated by B lymphocytes
- •Produces antibodies
- •Neutralizes extracellular pathogens/toxins
- •Uses helper T cells for activation
- •Key vs bacteria, toxins
Step-by-step worked examples
Why can't antibodies alone clear a virus that has already infected a cell?
Antibodies can only bind targets outside cells (extracellular) Once a virus is inside a host cell, antibodies cannot reach it Cytotoxic T cells recognize viral peptides displayed on MHC class I The infected cell is killed, stopping viral replication
A patient with a genetic defect that blocks T cell development frequently develops severe viral and fungal infections despite normal antibody levels. Explain why.
Normal antibody levels show B cells (humoral immunity) are working Without T cells, cytotoxic T cells cannot kill infected cells Helper T cells also can't activate macrophages or coordinate the response Cellular immunity fails, so intracellular pathogens (viruses, fungi) are not cleared
Why does organ transplant rejection primarily involve cellular immunity?
Transplanted cells carry foreign MHC molecules on their surface Recipient's cytotoxic T cells recognize these as non-self T cells directly attack and destroy the transplanted tissue cells This is why immunosuppressants targeting T cells (e.g. cyclosporine) prevent rejection
Flashcards
Quick quiz
Q1.Which cell type is central to cellular immunity?
Q2.Cytotoxic T cells recognize infected cells via which molecule?
Q3.Which immune branch does NOT rely on antibodies?
Q4.What do cytotoxic T cells release to kill target cells?
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Common mistakes
Thinking all adaptive immunity involves antibodies. — Correct: Cellular immunity (T cells) works without antibodies, directly killing infected cells.
Confusing helper T cells with cytotoxic T cells. — Correct: Helper T cells (CD4+) coordinate the response; cytotoxic T cells (CD8+) do the killing.
Believing T cells recognize free-floating pathogens. — Correct: T cells only recognize antigens presented on MHC molecules on cell surfaces, not free pathogens.
Assuming cellular immunity is innate. — Correct: Cellular immunity is part of the ADAPTIVE immune system — specific and has memory.
FAQ
What is cellular immunity?
Cellular immunity (cell-mediated immunity) is the branch of adaptive immunity where T lymphocytes directly destroy infected, cancerous, or foreign cells without using antibodies.
What are examples of cellular immunity?
Cytotoxic T cells killing virus-infected cells, T cells attacking tumor cells, and T cell-mediated transplant rejection are classic examples.
How is cellular immunity different from humoral immunity?
Cellular immunity uses T cells to kill cells directly; humoral immunity uses B cells to produce antibodies that neutralize pathogens outside cells.
Why is cellular immunity important in biology class?
It explains how the body clears viruses, cancer cells, and fungal infections that antibodies alone cannot reach — a key concept in immunology units.