**Assertion (A):** The female Anopheles mosquito is the definitive host of Plasmodium, while humans are the intermediate host. **Reason (R):** Sexual reproduction of Plasmodium occurs in the mosquito's gut, which is the criterion for designating the definitive host.

**Assertion (A):** A person vaccinated against measles as a child rarely develops measles upon re-exposure decades later. **Reason (R):** Vaccination stimulates production of memory B-cells and T-cells that rapidly mount a secondary immune response upon re-exposure, preventing clinical disease.

**Assertion (A):** HIV infection cannot be transmitted by casual contact such as handshaking or sharing utensils. **Reason (R):** HIV requires bodily fluids (blood, semen, vaginal secretions, breast milk) for transmission and cannot survive outside the human body for extended periods.

**Assertion (A):** Malignant tumors are more dangerous than benign tumors primarily because of metastasis. **Reason (R):** Metastatic cancer cells spread through blood and lymphatic circulation to establish secondary tumors at distant sites, making complete surgical removal impossible.

In a diagram of an immunoglobulin (antibody) molecule, the region labeled "Fab" refers to: