Which of the following best describes the concept of immunity?

Explanation: Macrophages are a type of white blood cell that plays a crucial role in the innate immune response. They are phagocytes, meaning they engulf and destroy pathogens, cellular debris, and foreign substances in the body. Macrophages are also antigen-presenting cells, as they process and present antigens to other immune cells to initiate an adaptive immune response.

Incorrect choices:

a. B cells produce antibodies against specific antigens but do not directly engulf and destroy pathogens.

b. T cells are involved in cell-mediated immunity and directly attack infected or abnormal cells but do not have phagocytic capabilities like macrophages.

d. Natural killer cells are a type of lymphocyte involved in the innate immune response and can directly kill infected or abnormal cells, but they do not primarily function as phagocytes like macrophages.

Explanation: Multiple sclerosis (MS) is an autoimmune disorder in which the immune system attacks the myelin sheath, the protective covering of nerve fibers in the central nervous system, leading to communication problems between the brain and the rest of the body.

Incorrect choices:

a. Rheumatoid arthritis primarily affects the joints, not the central nervous system.

b. Sjögren's syndrome primarily affects the salivary and lacrimal glands, not the central nervous system.

d. Crohn's disease is an autoimmune disorder that affects the gastrointestinal tract, not the central nervous system.