Which type of immune response is the body's first line of defense against infections?

Explanation: Helper T cells (CD4+ T cells) play a critical role in the immune response by enhancing the activity of other immune cells. They secrete cytokines that stimulate B cells to produce antibodies, activate cytotoxic T cells to kill infected cells, and promote the function of macrophages in phagocytosis. Helper T cells are essential for coordinating and regulating the immune response.

Incorrect choices:

a. Cytotoxic T cells (CD8+ T cells) are responsible for directly attacking and killing infected cells, not enhancing the activity of other immune cells.

b. Memory T cells are formed after the immune system encounters an antigen and are not directly involved in enhancing immune cell activity.

d. Suppressor T cells (regulatory T cells) are involved in down-regulating the immune response to prevent excessive inflammation and tissue damage, not in enhancing immune cell activity.

Explanation: Cytotoxic T cells (CD8+ T cells) are the effector cells of cell-mediated immunity. Their primary role is to directly recognize and kill infected or abnormal cells, such as virus-infected cells or cancer cells. Cytotoxic T cells release cytotoxic granules containing perforin and granzymes, which induce apoptosis (cell death) in the target cells.

Incorrect choices:

a. Helper T cells, not cytotoxic T cells, recognize antigens presented by macrophages and play a crucial role in activating other immune cells.

b. B cells are responsible for producing antibodies against pathogens during the humoral immune response, not cytotoxic T cells.

d. Regulatory T cells (suppressor T cells) are involved in regulating the immune response to prevent excessive inflammation and tissue damage, not cytotoxic T cells.