_____ fight against germs that may enter our body.

Concept: The immune system defends the body against germs (bacteria, viruses, and other pathogens) mainly through white blood cells (leukocytes), which circulate in the blood and are specialised to detect, engulf, and destroy invading microbes.

Application: The sentence asks for the blood component responsible for fighting germs that enter the body. Among the options given, white blood cells are the immune-defense cells that carry out this function, so “White blood cells” is the word that correctly completes the sentence.

• Red blood cells carry oxygen from the lungs to the body's tissues and bring back carbon dioxide; they have no role in fighting infection.

• Platelets are cell fragments that gather at a wound to help the blood clot and stop bleeding; they do not attack germs.

• Hemoglobin is the iron-containing protein inside red blood cells that binds and carries oxygen; it is not a cell and plays no part in immune defense.

Cross-check: Of the four options, only white blood cells are living immune cells built to recognise and destroy pathogens, which matches the function described in the sentence.