Which of the following is an example of passive immunity?

Explanation: B cells recognize antigens by producing specific receptors on their cell surface, known as antibodies or immunoglobulins. When a B cell encounters an antigen that matches its specific antibody, it binds to the antigen, marking it for destruction or neutralization by other immune cells.

Incorrect choices:

a. Engulfing and processing pathogens is the role of antigen-presenting cells like macrophages and dendritic cells. These cells present processed antigens to other immune cells, including B cells, to trigger an immune response.

b. Cytokines are small proteins that serve as signaling molecules in the immune system. They are produced by various immune cells to regulate immune responses, but they are not directly involved in antigen recognition by B cells.

d. Directly attacking and killing infected cells is the role of cytotoxic T cells during cell-mediated immunity, not B cells during the humoral immune response.

Explanation: T cells are a type of lymphocyte that plays a central role in cell-mediated immunity. They can recognize and destroy abnormal cells, including cancer cells, through direct cell-to-cell interactions.

Incorrect choices:

b. B cells produce antibodies in response to specific pathogens, but they do not directly attack cancer cells.

c. Antibodies are protein molecules that can bind to specific antigens on pathogens, but they do not have a direct role in attacking cancer cells.

d. Macrophages are phagocytic cells that engulf and destroy foreign substances, including pathogens, but they are not the primary cells responsible for recognizing and attacking cancer cells.