Defence Against Diseases

The pathogen is ingested — swallowed with contaminated food — so it travels through the digestive system. We need a first-line defence in the digestive tract.

The stomach contains hydrochloric acid at approximately pH 2, which kills most pathogens that are swallowed. This is a chemical barrier and part of the first line of defence.

Phagocytosis and antibody production are the second and third lines of defence respectively — they only activate after a pathogen has already entered the body. The question asks about killing the bacteria before they reach deeper tissues.

Stomach acid is the correct first-line chemical barrier for ingested pathogens. Cilia are found in the respiratory tract, not the digestive tract, so that option is also incorrect.

The question asks which first-line defence acts against bacteria that have been eaten with contaminated food, before they penetrate deeper tissues.

"Phagocytes engulfing the bacteria in the bloodstream" is the second line of defence — phagocytosis occurs after pathogens have already entered tissues or blood, not before entry.

"B lymphocytes releasing antibodies into the gut" is the third line of defence (adaptive immune system). This takes days to weeks and is not a first-line barrier.

"Cilia sweeping bacteria out of the respiratory tract" is a first-line defence, but cilia are in the respiratory tract, not the digestive tract. This does not apply to ingested bacteria.

Stomach acid at pH ~2 is the correct answer. It is a first-line chemical barrier in the digestive system that kills most swallowed pathogens before they can cause infection.

Antibiotics are drugs that kill or inhibit bacteria. They work by targeting structures or processes that are unique to bacteria — such as bacterial cell walls and ribosomes — that are absent or different in human cells.

Streptococcus is a bacterium, so it has a cell wall and bacterial ribosomes. An antibiotic like penicillin disrupts cell wall synthesis, killing the bacterium without harming the patient's own cells.

Antibiotics do not destroy viral capsids (bacteria don't have capsids — that's a virus structure), do not directly stimulate B cells, and do not raise body temperature (that is the fever response).

The correct explanation is that antibiotics target bacterial-specific structures like cell walls and ribosomes, making them effective against bacteria but harmless to human cells.

The question asks for the correct explanation of why antibiotics work against a bacterial infection. We need to find the option that accurately describes the mechanism of antibiotics.

"Antibiotics destroy the viral capsid" is incorrect — bacteria are prokaryotic cells, not viruses. They do not have a capsid. Capsids are protein coats found only on viruses.

"Antibiotics stimulate B cells to produce antibodies" is incorrect. Antibiotics are not immunostimulants — they act directly on bacteria, not on the immune system.

"Antibiotics raise body temperature" is incorrect. Fever is caused by pyrogens released by white blood cells — it is part of the second-line immune response, not an effect of antibiotics.

Antibiotics target bacterial structures such as cell walls and ribosomes is correct. These features exist in bacteria but not in human cells, allowing antibiotics to selectively kill bacteria.