Answer: T cells require BOTH signal 1 (TCR binding MHC-peptide) AND signal 2 (CD28 binding B7/CD80/86 on APCs); signal 1 alone causes anergy, not activation.
- A T cells are sometimes thought to need only one signal to activate, with TCR engagement of MHC-peptide alone being sufficient for full activation
- B T cells require BOTH signal 1 (TCR binding MHC-peptide) AND signal 2 (CD28 binding B7/CD80/86 on APCs); signal 1 alone causes anergy, not activation
- C Two signals are sometimes thought to cause cell death of the T cell rather than driving its proliferation and full effector differentiation process
- D Costimulation is sometimes thought to be mainly for B cells, despite CD28 binding to B7 occurring specifically and mainly on T lymphocytes
Correct answer: B. T cells require BOTH signal 1 (TCR binding MHC-peptide) AND signal 2 (CD28 binding B7/CD80/86 on APCs); signal 1 alone causes anergy, not activation
Explanation: T cell activation requires two signals: (1) TCR recognizes specific MHC-peptide (antigen-specific). (2) CD28 binds B7 (CD80/86) on professional APCs (costimulation). Signal 1 alone = anergy (unresponsiveness). This prevents inappropriate T cell activation against self-antigens presented by non-APCs lacking B7.
An antibody is Y-shaped, with two identical antigen-binding sites at the tips of the variable regions - each shaped to recognise one specific antigen - while the constant (Fc) region at the base is recognised by immune cells to trigger further responses.
Concept context
Innate and adaptive immunity, B cells, T cells, antibodies, vaccines, and immune disorders. Critical for NEET.