Immune System

Cellular barriers of immunity include cells like neutrophils, macrophages, and monocytes. These cells play key roles in defending the body from pathogens. Cytokines also help in immune responses but are signaling molecules, not cellular barriers themselves. Saliva and lysozyme are involved in non-cellular immune barriers.
Hence, the correct answer is option (4) b, c, f.

The thymus gland is the primary lymphoid organ where T-lymphocytes (T-cells) mature and differentiate. It plays a critical role in the immune system, unlike the spleen, lymph nodes, and tonsils, which are secondary lymphoid organs involved in filtering lymph and providing immune responses.