Answer: Skin and mucous membranes.
- A Antibodies circulating in the blood plasma
- B Skin and mucous membranes
- C T lymphocytes patrolling the lymphatic system
- D Fever raising the body's core temperature
Correct answer: B. Skin and mucous membranes
Explanation: The first line of defence includes physical and chemical barriers: skin (prevents entry), mucus (traps pathogens), cilia (sweep pathogens out), saliva, tears (contain lysozyme), and stomach acid.
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.