cytokines

Meaning: Cytokines are small secreted proteins that bind specific receptors on target cells and change their behavior, such as gene expression, growth, or movement. Immune cells like macrophages and helper T cells are major sources, though many other cell types also make them. Most act over short range, either on neighboring cells (paracrine) or on the secreting cell itself (autocrine); some reach distant targets through the blood. Examples include interleukins, interferons, and tumor necrosis factor. Their jobs include recruiting leukocytes, driving T and B cell proliferation, promoting inflammation and fever, and triggering antiviral defenses.

On the MCAT: A passage describes an infection or immune response in which macrophages release cytokines that recruit neutrophils and increase capillary permeability, or helper T cells secrete cytokines that activate B cells and cytotoxic T cells. You may be asked to predict the result of blocking a cytokine or its receptor (a weaker or missing downstream response), or to connect cytokines to receptor signaling and cell communication.

Common confusion: Cytokines are not antibodies and are not classic hormones. Antibodies are made by plasma cells and bind antigens directly; cytokines bind receptors on host cells and carry messages. Classic hormones come from endocrine glands and travel in the blood, while cytokines come from many cell types and often act locally.