In the following passage, each numbered space can be filled with a biological term.
When a pathogen invades the body, phagocytes engulf them and present their foreign (1) ___________ on their own cell surface membrane. These cells are now called (2) ___________ cells. Specific T-lymphocytes called T-helper cells have receptor proteins that bind to these presented molecules. Once activated, these T-helper cells stimulate B-cells and secrete (3) ___________, which are chemical signaling proteins. Cytotoxic T-cells are also stimulated to destroy infected host cells by releasing a protein called (4) ___________, which makes holes in the cell membrane. This direct cellular defense is a key component of the cell-mediated response, whereas the transfer of pre-formed antibodies to an individual provides immediate but temporary (5) ___________ immunity. Long-term immunity relies on the production of (6) ___________ cells that remain in the blood to respond rapidly to future infections.
Write the correct biological term beside each number from 1 to 6 that matches the space in the passage.
Developing an effective vaccine against the Human Immunodeficiency Virus (HIV) has proven extremely difficult. Suggest one reason why, relating to the antigenic properties of the virus.
Table 1 shows the action of two antimicrobial agents.
| Antimicrobial agent | Action of antimicrobial |
|---|---|
| Penicillin | Inhibits enzymes involved in cross-linking peptidoglycan chains in bacterial cell walls. |
| Streptomycin | Binds to 70S ribosomes, preventing translation in bacterial protein synthesis. |
Using the information in Table 1, explain why each antimicrobial agent is not effective against HIV.