Which type of blood vessel is primarily responsible for the exchange of nutrients and waste products between blood and tissues?

A. Hinge joint: Hinge joints allow movement primarily in one plane, such as flexion and extension, like the elbow or knee, limiting rotation and side-to-side motion.

B. Saddle joint: Saddle joints permit movement in two planes, such as flexion-extension and abduction-adduction, but do not allow full rotational movement.

C. Ball and socket joint: Ball and socket joints, like the shoulder and hip, have a spherical head fitting into a cup-like cavity, allowing multiaxial movement including flexion, extension, abduction, adduction, and rotation.

D. Pivot joint: Pivot joints allow rotational movement around a single axis, such as the atlantoaxial joint, but do not permit flexion or abduction.

A. Activating B cells and other immune cells: Helper T cells coordinate the immune response by releasing cytokines that stimulate B cells to produce antibodies and activate cytotoxic T cells and macrophages, enhancing both humoral and cell-mediated immunity.

B. Producing antibodies: Antibodies are produced by plasma cells, which are differentiated B cells, not by helper T cells.

C. Directly killing infected cells: Cytotoxic T cells are responsible for directly attacking and destroying infected or abnormal cells, whereas helper T cells regulate and support the immune response.

D. Engulfing pathogens: Phagocytic cells such as macrophages and neutrophils perform pathogen engulfment; helper T cells do not directly ingest pathogens.