# How Two Special Antibodies Find Substances in Body Fluids > This patent describes a "sandwich" method using two highly specific, man-made antibodies to detect and measure tiny amounts of specific substances, like disease markers, in a fluid sample. - **Patent:** US 4376110 - **Original title:** Immunometric assays using monoclonal antibodies - **Owner:** Hybritech Inc - **Granted:** 1983 - **Status:** Public domain (expired) - **Times cited:** 1,572 - **Field:** biotech, pharmaceutical, diagnostics, medical_devices ## What it does This patent describes a "sandwich" method to find or measure tiny amounts of a specific substance, called an "antigenic substance," in a fluid. First, a sample of the fluid is mixed with a measured amount of a special, man-made "first monoclonal antibody" that has a label, like a radioactive tag (Claim 1(a), Claim 8). This first antibody attaches to the target substance. Then, this mixture is exposed to a "second monoclonal antibody" that is stuck to a solid surface (Claim 1(b)). This second antibody also attaches to the target substance, creating a "sandwich" where the target substance is held between the two antibodies. The solid surface is then separated and washed (Claim 1(c), Claim 5), and the amount of labeled first antibody stuck to it is measured (Claim 1(d)). By comparing this measurement to known samples, the presence or amount of the target substance can be determined (Claim 1(e)). For example, this could be used to detect hepatitis B proteins in a blood sample. ## What it does NOT cover - Does not cover assays that use polyclonal antibodies, which are mixtures of different antibodies. - Does not cover single-antibody detection methods or competitive binding assays. - Does not cover methods where both antibodies are soluble or both are bound to a solid surface. - Does not cover antibodies with an affinity lower than 10^8 liters/mole for the target substance. ## The clever bit The clever bit was combining the newly available, highly specific "monoclonal antibodies" with the "two-site sandwich" assay format. This allowed for much more precise and reliable detection of tiny amounts of specific substances, reducing false positives and improving consistency compared to older methods using less specific antibody mixtures. ## Real-world examples 1. Home pregnancy tests 2. Rapid strep throat tests 3. Tests for specific cancer markers like CEA or alphafetoprotein 4. Blood tests for hepatitis B 5. Tests for thyroid stimulating hormone (TSH) levels ## Why it matters This patent was filed early in the development of monoclonal antibody technology, applying these highly specific tools to the widely used "sandwich" immunoassay format. This combination significantly improved the accuracy and sensitivity of diagnostic tests for various substances in biological fluids. It laid a foundation for more precise detection of disease markers, hormones, and other critical molecules in clinical diagnostics. ## Frequently asked questions ### What does How Two Special Antibodies Find Substances in Body Fluids cover? This patent describes a "sandwich" method using two highly specific, man-made antibodies to detect and measure tiny amounts of specific substances, like disease markers, in a fluid sample. ### Who owns patent US 4376110? Hybritech Inc owns this patent, granted in 1983. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 4376110 cited by? This patent has been cited by 1572 later patents that build on its ideas. ### What problem does this patent solve? This patent was filed early in the development of monoclonal antibody technology, applying these highly specific tools to the widely used "sandwich" immunoassay format. This combination significantly improved the accuracy and sensitivity of diagnostic tests for various substances in biological fluids. It laid a foundation for more precise detection of disease markers, hormones, and other critical molecules in clinical diagnostics. ### What does this patent NOT cover? Does not cover assays that use polyclonal antibodies, which are mixtures of different antibodies. **Full plain-English explainer:** https://patentbrief.org/patent/us/4376110/psa-test-for-prostate-cancer **Original patent:** https://patents.google.com/patent/US4376110 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How to Detect Disease Markers Using Colored Proteins](https://patentbrief.org/patent/us/4302536/colorimetric-immunoassay-process) — A 1981 method for measuring disease-related proteins in blood by attaching them to naturally colored proteins that can be detected with light. - [Using Radioactive Antibodies to Find Tumors Inside the Body](https://patentbrief.org/patent/us/3927193/ivermectin) — A 1973 method for finding cancer tumors by injecting patients with radioactive antibodies that attach to tumor-specific proteins, allowing doctors to scan for their location. - [How to Build Smaller, Synthetic Antibody-Like Molecules for Medicine](https://patentbrief.org/patent/us/5455030/immunotheraphy-using-single-chain-polypeptide-binding-molecules) — This patent describes a way to create small, single-chain proteins that mimic the binding power of full-sized antibodies to deliver medicine more effectively. - [How Roche Engineers Antibodies to Target Two Different Diseases Simultaneously](https://patentbrief.org/patent/us/10138293/opdivo-nivolumab) — A method for creating a custom-engineered antibody that can grab onto two different targets at once, designed to help the immune system fight complex diseases. - [Making Hybrid Antibodies from Different Animals](https://patentbrief.org/patent/us/4816567/recombinant-antibody-production-in-cho-cells) — This patent describes how to create new, engineered antibodies by combining parts of antibodies from two different animal species, then growing them in a lab.