What are the components of the immune system?

• A. Skin, mucous membranes, lymphoid tissue (spleen, thymus, lymph nodes), and bone marrow.
• B. Heart, lungs, liver, and kidneys.
• C. Muscles, bones, and connective tissue.
• D. Endocrine glands and nervous tissue.

Answer: (A)

The immune system relies on protective barriers plus organized lymphoid tissues where immune cells develop and respond. The skin and mucous membranes form the first line of defense, blocking entry and producing antimicrobial substances. Inside the body, bone marrow generates immune cells, including B cells, while the thymus educates T cells. Lymphoid tissues such as the spleen and lymph nodes act as hubs where immune cells meet pathogens, become activated, and coordinate adaptive responses. These components together—barriers plus lymphoid organs—define the immune system. Other organs listed (heart, lungs, liver, kidneys) or tissues (muscle, bone, endocrine, nervous tissue) are not primary immune-system components.