What is the primary function of the alveoli in the respiratory system?

A. T cells: T cells are primarily involved in cell-mediated immunity. They help regulate immune responses and destroy infected or abnormal cells but do not directly produce antibodies.

B. B cells: B cells are responsible for humoral immunity and differentiate into plasma cells that secrete antibodies. These antibodies bind to specific antigens to neutralize or mark them for destruction by other immune cells.

C. Macrophages: Macrophages are phagocytic cells that engulf pathogens and present antigens to lymphocytes. They support the immune response but do not produce antibodies themselves.

D. Natural killer cells: Natural killer cells destroy infected or cancerous cells through cytotoxic activity. They are part of innate immunity and do not generate antibodies.

A. Metallic bond: Metallic bonds involve the delocalized sharing of electrons among a lattice of metal atoms, providing conductivity and malleability, but not discrete electron pair sharing between specific atoms.

B. Hydrogen bond: Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to one electronegative atom and another electronegative atom; they do not involve direct electron pair sharing.

C. Covalent bond: Covalent bonds form when two atoms share one or more pairs of electrons to achieve stable electron configurations, creating strong and specific molecular connections.

D. Ionic bond: Ionic bonds result from the electrostatic attraction between oppositely charged ions following electron transfer, rather than sharing electron pairs.