Which of the following is NOT a component of the immune system?

Explanation: Antigens are unique molecules or proteins present on the surface of cells, including pathogens. The immune system can recognize these antigens as "non-self" and mount an immune response to eliminate them while sparing the body's own cells, which display different antigens that are recognized as "self."

Incorrect choices:

b. Recognizing different blood types is relevant for blood transfusions but not a primary mechanism of distinguishing self from non-self by the immune system.

c. Monitoring hormone levels in the body is not directly related to the immune system's ability to distinguish between self and non-self.

d. Cytokines are signaling molecules released by immune cells to regulate the immune response but are not the primary means of distinguishing self from non-self.

Explanation: Hormones are chemical messengers produced by the endocrine system to regulate various physiological processes in the body. While the endocrine system and immune system can influence each other's activities, hormones are not considered a direct component of the immune system.

Incorrect choices:

a. Lymphocytes are a type of white blood cell and a critical component of the immune system. They include B cells, T cells, and natural killer cells, which play essential roles in immune responses.

b. Antigens are molecules or substances that can trigger an immune response in the body. They are recognized by the immune system as foreign or non-self and can elicit the production of antibodies.

d. Antibodies are proteins produced by B cells in response to the presence of antigens. They play a vital role in recognizing and neutralizing foreign substances in the body.