All lymphocytes originate in the ______.

• A. thymus
• B. bone marrow
• C. spleen
• D. lymph nodes

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the question. The problem is asking where all lymphocytes originate, which is a fundamental concept in immunology and hematopoiesis.
2. Step 2: Recall the biological process of lymphocyte development. Lymphocytes are a type of white blood cell involved in the immune response. They include B cells, T cells, and natural killer (NK) cells.
3. Step 3: Identify the site of origin for lymphocytes. All blood cells, including lymphocytes, originate from hematopoietic stem cells, which are found in the bone marrow.
4. Step 4: Clarify the role of other organs mentioned in the options. The thymus is where T cells mature, the spleen is involved in filtering blood and immune responses, and lymph nodes are sites for immune activation, but none of these are the site of origin for lymphocytes.
5. Step 5: Conclude that the correct answer is 'bone marrow,' as it is the primary site of hematopoiesis and the origin of all lymphocytes.