Which of the following types of vaccines involves the use of a weakened form of the pathogen?

Correct answer: B

Explanation: T cells, specifically cytotoxic T cells (CD8+ T cells), are responsible for cell-mediated immunity. They directly recognize and attack infected or abnormal cells, leading to their destruction. This process is crucial in controlling viral infections and cancer cells.

Incorrect choices:
a. B cells, not T cells, are responsible for the production of antibodies during the humoral immune response.
c. Phagocytosis of pathogens is mainly carried out by macrophages and neutrophils, not T cells.
d. Allergic reactions are primarily mediated by a different type of T cells called helper T cells (CD4+ T cells), not cytotoxic T cells.

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.