Which of the following is true of leucocytes?

Step 1: Analyze the question Leucocytes are white blood cells. We need the statement that correctly describes them.

Step 2: Recall the concept Blood has three main cellular components:

• Erythrocytes (red blood cells): contain haemoglobin (a respiratory pigment), are the most numerous, and in mammals have no nucleus.
• Leucocytes (white blood cells): larger than red cells, have a nucleus, and defend the body against infection (phagocytosis and antibody production).
• Thrombocytes (platelets): cell fragments involved in blood clotting.

Step 3: Evaluate each option

• A. They are respiratory pigments. False. A respiratory pigment is haemoglobin, found in red blood cells. Leucocytes are cells, not pigments.
• B. They are most numerous and ramify all cells. False. Red blood cells are the most numerous. Leucocytes are comparatively few (about 1 white cell to roughly 600-700 red cells).
• C. They are large and nucleated. True. Leucocytes are bigger than red blood cells and each has a distinct nucleus (lobed in granulocytes, rounded in lymphocytes).
• D. They are involved in blood clotting. False. This is the job of platelets (thrombocytes), together with clotting factors such as fibrinogen.

Step 4: Answer The correct option is C.

Exam tip: Remember "WBC = Warriors, Big and with a nucleus" (they fight infection, are bigger than RBCs, and have nuclei). RBC carries oxygen, platelets clot blood.