Cytokines and Interleukins Overview
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Direct answer
Interleukins are a large family of cytokines, secreted proteins that let leukocytes signal one another during innate and adaptive immune responses. Key examples include IL-1 and TNF-α for inflammation, IL-12 and IFN-γ for Th1 immunity, IL-4 for Th2 responses and IL-2 for T cell proliferation. Each cytokine has a characteristic cellular source and a dominant biological effect, and examiners test both.
What you must remember
- IL-1 and TNF-α: the early inflammatory pair from macrophages, producing fever, endothelial activation and acute phase changes, with TNF-α central to septic shock.
- IL-2: the T cell growth factor secreted by activated T cells, driving clonal expansion and supporting regulatory T cell survival.
- IL-4, IL-5 and IL-13: Th2 cytokines that drive IgE switching, eosinophil activation and mucus secretion, central to allergy and helminth defence.
- IL-12 and IFN-γ: the Th1 axis, in which IL-12 from dendritic cells drives IFN-γ production that activates macrophages against intracellular pathogens.
- IL-17: the Th17 cytokine that recruits neutrophils and drives antimicrobial responses at mucosal surfaces.
- Type I interferons: IFN-α and IFN-β, made by virus-infected cells, which induce an antiviral state in neighbouring cells.
- IL-10 and TGF-β: inhibitory cytokines that dampen Th1 and macrophage responses and underpin regulatory T cell function.
Matching each cytokine to its source and effect
Cytokines are easiest to remember as a grid of source and effect, because most exam questions give one and ask for the other. The table lists the cytokines most often tested in NEET-PG Microbiology.
| Cytokine | Main source | Key function |
|---|---|---|
| IL-1 | Macrophages | Fever, endogenous pyrogen, local inflammation |
| IL-2 | Activated T cells | T cell growth and proliferation |
| IL-4 | Th2 cells | IgE switching, Th2 differentiation |
| IL-6 | Macrophages, T cells, endothelium | Acute phase response, fever, B cell differentiation |
| IL-10 | Th2 cells, regulatory T cells, macrophages | Anti-inflammatory, suppresses Th1 responses |
| IL-12 | Dendritic cells, macrophages | Drives Th1 differentiation and IFN-γ production |
| IFN-γ | Th1 cells, NK cells | Activates macrophages, raises MHC class II expression |
| TNF-α | Macrophages | Fever, septic shock, granuloma formation, cachexia |
The inflammatory cytokines IL-1, IL-6 and TNF-α share a common macrophage origin and an acute phase effect. IL-12 and IFN-γ work as a pair for cell-mediated defence, while IL-4 and IL-10 sit on the opposing humoral and inhibitory side.
Where the exam tries to catch you
Cytokine questions often mix up a source with a function, so checking both halves of each pairing prevents most errors.
- Trap: attributing fever to IL-2 Correct: IL-1, IL-6 and TNF-α are the pyrogenic cytokines, whereas IL-2 is the T cell growth factor.
- Trap: listing IFN-γ as a type I interferon Correct: IFN-γ is a type II interferon from T and NK cells, whereas IFN-α and IFN-β are type I and antiviral.
- Trap: linking IL-4 with cell-mediated immunity Correct: IL-4 drives humoral Th2 responses and IgE switching, whereas IL-12 and IFN-γ drive cell-mediated Th1 immunity.
Frequently asked questions
Which cytokine is the classic endogenous pyrogen?
Interleukin-1 is the classic endogenous pyrogen, released mainly by macrophages and acting on the hypothalamus to raise body temperature. Interleukin-6 and TNF-α also contribute to fever, which is why the three are grouped together as pyrogenic cytokines.
Which interleukin is known as the T cell growth factor?
Interleukin-2 is known as the T cell growth factor because activated T cells secrete it to drive their own proliferation. Its receptor, CD25, is also present on regulatory T cells, which depend on IL-2 for survival and suppressive function.
Which cytokines drive the Th2 response in allergy?
IL-4, IL-5 and IL-13 drive the Th2 response in allergy. IL-4 promotes IgE class switching, IL-5 supports eosinophil production and activation, and IL-13 contributes to airway mucus secretion and bronchial hyper-responsiveness.
What is the difference between type I and type II interferons?
Type I interferons include IFN-α and IFN-β, which are secreted by virus-infected cells and induce an antiviral state in neighbouring cells. The only type II interferon is IFN-γ, which is secreted by Th1 and NK cells and mainly activates macrophages.