Answer: Cell surface proteins that present antigen fragments to T cells; essential for T cell activation and self/non-self recognition.
- A A specific type of antibody molecule that is secreted into general circulation by activated plasma cells according to conventional understanding
- B Cell surface proteins that present antigen fragments to T cells; essential for T cell activation and self/non-self recognition
- C A type of complement protein that is sometimes thought to activate specifically during the alternative pathway in routine practice
- D The receptor protein found on B cell surfaces that circulating antibodies are generally designed to bind overall in most cases
Correct answer: B. Cell surface proteins that present antigen fragments to T cells; essential for T cell activation and self/non-self recognition
Explanation: MHC molecules are cell surface glycoproteins. MHC class I (on all nucleated cells) presents intracellular antigens to CD8<sup>+</sup> cytotoxic T cells. MHC class II (on antigen-presenting cells) presents extracellular antigens to CD4<sup>+</sup> T helper cells. MHC determines transplant compatibility (HLA in humans).
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.