Integrins are a family of proteins that sit on the surface of nearly every cell in your body, acting as the primary link between a cell’s interior and its surrounding environment. Humans have 24 distinct integrins, each assembled from a pair of two smaller protein chains called alpha and beta subunits (18 alpha types and 8 beta types combine in specific pairings). These receptors do far more than anchor cells in place. They relay signals in both directions across the cell membrane, influencing everything from wound healing and immune defense to cancer spread.
How Integrins Are Built
Every integrin is a two-part molecule, a pairing of one alpha subunit and one beta subunit that together form what biologists call a heterodimer. Neither subunit works alone. The combination determines which molecules outside the cell the integrin can grab onto and what signals it sends inside. An integrin made from the alpha-1 and beta-1 subunits, for example, attaches to collagen and laminin, while one made from alpha-5 and beta-1 binds fibronectin.
Each integrin has a shared architecture: a “head” region that extends outward to contact surrounding proteins, leg-like segments that span the space between the cell surface and the head, and a short tail that dips into the cell’s interior. That tail is where intracellular machinery latches on to control the integrin’s behavior.
What Integrins Bind To
Outside the cell, integrins connect to proteins in the extracellular matrix, the structural scaffold that fills the space between cells in tissues. The list of binding partners is long: collagens, fibronectin, laminin, vitronectin, fibrinogen, thrombospondins, and von Willebrand factor, among others. Fibronectin alone can be recognized by at least 12 different integrins, making it one of the most versatile integrin targets. Collagen binds a narrower set, while laminin is recognized by six.
This variety means different cell types can use different integrins to interact with the same tissue environment in distinct ways. A skin cell, a platelet, and an immune cell may all encounter fibronectin, but each responds differently depending on which integrins it carries.
Bidirectional Signaling
One of the most distinctive features of integrins is that they transmit information in two directions. Signals can travel from inside the cell outward (inside-out signaling) or from the external environment inward (outside-in signaling). These two pathways are separate but influence each other.
In inside-out signaling, a signal originating within the cell causes the integrin to change shape so its outer head becomes “stickier” and binds external targets more readily. The key player here is a protein called talin, which binds to the integrin’s beta subunit tail inside the cell. This loosens the grip between the alpha and beta subunits at their base, allowing the integrin’s external head to open up and grab ligands. A second protein, kindlin, assists talin by binding a nearby site on the same tail.
Outside-in signaling works in reverse. When an external molecule docks with the integrin’s head, the integrin unfolds and its two subunits separate, transmitting a structural change through the membrane into the cell. This triggers a cascade of internal signals, most notably through a signaling hub called focal adhesion kinase (FAK). Once activated, FAK recruits other signaling molecules that influence cell survival, movement, and growth.
The On/Off Switch
Integrins are not always active. In their resting state, the two subunits are clasped together and the extracellular head is bent downward toward the cell surface, unable to engage ligands effectively. Activation requires disrupting that clasp, and several proteins compete to either flip the switch on or keep it off.
Talin and kindlin are the primary activators. But other proteins act as brakes. Filamin, an actin-linking protein, binds the same region on the beta tail that talin uses, blocking activation. Another inhibitor, ICAP1, occupies the kindlin-binding site and physically prevents both kindlin and talin from attaching. On the alpha subunit side, proteins like SHARPIN stabilize the resting clasp between the two subunits. The balance between these activators and inhibitors determines whether an integrin is open for business at any given moment.
Integrins in Blood Clotting
Platelet aggregation, the clumping of blood cells that stops bleeding, depends entirely on a single integrin called alphaIIb-beta3. This integrin acts as a receptor for fibrinogen, the protein that crosslinks platelets into a clot. Without functional alphaIIb-beta3, platelets cannot aggregate at all. People born with defects in this integrin have a bleeding disorder called Glanzmann thrombasthenia.
What makes this integrin unusual is how tightly its activity is controlled. On resting platelets, alphaIIb-beta3 can stick to proteins that are already fixed to a surface, but it will not grab soluble fibrinogen floating in the bloodstream. Only after the platelet is activated by injury signals does the integrin shift into a high-affinity state capable of binding soluble fibrinogen and von Willebrand factor. This prevents dangerous clots from forming spontaneously while still allowing rapid clot formation at wound sites.
Integrins in Immune Cell Recruitment
When tissues become inflamed or infected, immune cells in the bloodstream need to find their way out of blood vessels and into the affected tissue. Two integrins are central to this process: VLA-4 (alpha4-beta1) and LFA-1 (alphaL-beta2). Both are widely expressed on immune cells.
The migration happens in stages. Immune cells first roll along the inner wall of blood vessels, then firmly adhere, and finally squeeze between the cells lining the vessel to enter the tissue. VLA-4 and LFA-1 participate in both the rolling and firm adhesion steps. VLA-4 plays a particularly broad role. It is critical for recruiting natural killer cells and NKT cells to the bone marrow, and T cell recruitment to the lungs during bacterial pneumonia depends on VLA-4 alone, with no contribution from LFA-1. Neutrophils, by contrast, rely on a combination of VLA-4 and a related integrin called Mac-1.
Integrins in Cancer
Tumor cells frequently alter which integrins they produce, and these changes can drive cancer progression. Increased expression of certain integrins correlates with poor prognosis across multiple cancer types. The mechanisms vary: some integrins cooperate with growth factor receptors to amplify pro-growth signals, others help tumor cells invade surrounding tissue by working with enzymes that break down the extracellular matrix.
One integrin in particular, alphav-beta3, has been linked to a cancer stem cell phenotype. Higher levels of this integrin correlate with resistance to anoikis, a form of cell death that normally occurs when cells detach from their surroundings. This resistance allows tumor cells to survive in the bloodstream as they travel to distant organs. Integrins also help determine where metastatic cells settle, because the integrin profile of a circulating tumor cell influences which tissues it can adhere to and colonize.
Drugs That Target Integrins
Because integrins play such specific roles in disease, they have become drug targets. Two integrin-blocking medications are FDA-approved for inflammatory bowel disease. Natalizumab, approved for Crohn’s disease in 2008, blocks the alpha4 integrin to prevent immune cells from migrating into the gut lining. Vedolizumab, approved in 2014 for both Crohn’s disease and ulcerative colitis, targets the alpha4-beta7 integrin more selectively. Vedolizumab has become the preferred option because its gut-specific targeting gives it a more favorable safety profile compared to the broader immune suppression caused by natalizumab. Natalizumab is also used in multiple sclerosis, where it reduces immune cell entry into the brain.