Interleukins fall into which class of signaling molecules?

• A. Neurotransmitters
• B. Cytokines
• C. Hormones
• D. Enzymes

Answer: (B)

Interleukins are signaling proteins produced by leukocytes that regulate the behavior of other immune cells. They belong to the cytokine family, a broad group of small secreted proteins that coordinate immune responses and inflammation. Their action is usually local (autocrine or paracrine), though some cytokines can have wider systemic effects. They bind specific receptors on target cells and trigger intracellular signaling pathways (like JAK-STAT) that change gene expression, promoting cell growth, differentiation, activation, or suppression as needed. This distinguishes them from neurotransmitters, which convey signals in the nervous system across synapses; from hormones, which are endocrine messengers released into the bloodstream by glands; and from enzymes, which are catalysts that speed chemical reactions rather than serve as signaling molecules. So interleukins are cytokines.