This invention describes a specific, sensitive, reproducible and multiplexed assay for simultaneous quantification of full-length beta-amyloid peptides in biological matrices; a method to monitor and measure the hallmarks associated with the progress of Alzheimer's disease. This invention employs one specific antibody that recognizes the N-terminus of all the abeta peptides, and a panel of detection antibodies that distinguish each abeta peptide by their sequence difference at the C-terminus. Each of these C-terminus specific antibodies carries a different label (or tag), for example, fluorescence labels with different excitation/emission characteristics or electrochemiluminescence (ECL) label with different excitation/emission characteristics. This invention allows simultaneous quantification of several abeta peptides in one single assay. The concept of using N- and C-terminus specific antibodies to capture the each of the said abeta peptides by both ends is the basis of providing the desired specificity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the two dimensional amino acid chain structure of Abeta amyloid peptide (1-42), FIG. 1 b , where the left side area is called N-terminus and the right side area is called C-terminus. Abeta peptide variants (1-39), FIG. 1 e , (1-40), FIG. 1 d , (1-41), FIG. 1 c , and (1-43), FIG. 1 a , have the similar amino acid chain structure as (1-42), FIG. 1 b , has, but have different number of amino acid in their prospective chain. Particular antibodies have been identified specifically for either the N, or C-terminus of each of Abeta peptide variants. For example, one is the mouse monoclonal antibody, such as clone number BAM-10 that recognizes the amino acids 1-12 (N-terminus) of an Abeta peptide, and an affinity purified antibody that recognizes the C-terminus of the Abeta (1-42). FIG. 2 illustrates a three-staged multiplexed immuno assay for beta amyloid peptides. In Stage 1 of FIG. 2 , Abeta(1-39), Abeta(1-40), Abeta(1-42) and Abeta(1-43), 20,25,30 and 35, respectively, are in microsphere. In stage 2 of FIG. 2 , the N-terminus specific antibody, 40, will bind and capture Abeta peptides(1-39), (1-40), (1-42) and Abeta (1-42), 20,25,30 and 35, respectively. In stage 3, different C-terminus specific antibodies, 45, 50, 55 and 60, each labeled with different fluorescence tags or chemiluminesence tags, are utilized 45-60 as another screen tool, whereby various Abeta peptides can be further characterized and distinguished. As long as the binding of an N-terminus specific antibody to the Abeta peptide does not exclude the binding of a C-terminus specific antibody to the Abeta peptide, use of both kinds of antibody can be a powerful screen tool to identify and quantize Abeta peptide having both C- and N-terminus. This invention only detects intact, full-length Abeta peptide (and/or its COOH terminal variants), but not its biologically distinguishable fragments or its aggregates (fibrils, or any polymeric forms). Several experimental results support the invention. For example, in a sample mixture of Abeta peptide (1-40) or (1-42) which is first run through an ELISA plate which is coated with the n-terminus specific antibody, both Abeta peptides (1-40) and (1-42) are captured by the N-terminus specific antibody. Next a C-terminus specific antibody, such as affinity purified rabbit polyclonal anti-Abeta(1-40): PharMingen lot number M029157, labeled with fluorescence (pretreated excitation at 485 nm frequency emission with 538 nm frequency) is applied to the captured Abeta peptides. The C-terminus antibody reacts with fluorescence—Ex succinimidyl estex then using any exhaust dislysis to remove excess fluorescence—Ex succinimidyl ester. FIG. 3 shows the result of this process. The x axis indicates the intensity of fluorescence which in turn directly correlating to the concentration of N-terminus specific antibody captured Abeta peptide bound with the fluorescence labeled C-terminus specific antibody. The data shows the concentration of captured Abeta(1-40), histogram 66, bound with the particular fluorescence labeled C-terminus antibodies, such as affinity purified rabbit polyclonal anti-Abeta(1-42): PharMingen lot number M050781 is almost close to undetectable level. This process does capture Abeta(1-42). The result is supported by the histogram 68 shown in FIG. 3 . If the sequence of applying N- and C-terminus specific antibodies is reversed, by first, applying fluorescence labeled C-terminus antibody to the (1-40) and (1-42) mixture sample and next running the resulting complex through the N-terminus specific antibody coated ELISA plate, this C- first then N- next sequence of applying antibodies significantly increases the amount of captured Abeta(1-42) as much as six (6) fold comparing to the N- first the C- next sequence. This indicates that the order of applying different C- and N-specific antibodies to the sample has different effects. The result is supported by histogram 70. In another experiment, shown in FIG. 4 , the above process is applied to two type samples, one with Abeta(1-42), only, and another with the same concentration of Abeta(1-42) plush Abeta(1-40). Referring to FIG. 4 , the histogram 76 shows the Abeta(1-42) only sample has the same fluorescence intensity level as the sample of mixture of Abeta&excl; 1-42) and (1-40) in histogram 78. The logical explanation for the results of FIG. 4 is that this invention will reliably measure the concentration of Abeta(1-42) in the sample with or without the presence of other Abeta peptides. The invention further demonstrates that the above process works on sample extracted from human tissue. FIG. 5 shows that the Abeta(1-42) sample in buffer, histogram 84, has the same fluorescence intensity level as the Abeta(1-42) sample in human plasma, histogram 86. The data shown in FIG. 5 , supports that the invention can reliably measure the concentration of Abeta(1-42) from a human plasma. Working with the same N-terminus specific antibody, the invention identifies another C-terminus specific antibody, such as affinity purified rabbit polyclonal anti-Abeta(1-40): PharMingen lot number M029157, labeled with Texas red (excitation 584, emission 612) for Abeta(1-40). The Texas Red labeled antibody was prepared by reacting the anti Abeta(1-40) antibody with Texas Red-X succinimidyl ester, excess Texas Red-x succinimidyl ester was removed by exhausted dialysis. A sample containing Abeta peptides was run through a N-terminus specific antibody coated ELISA plate. Abeta(1-40) and Abeta(1-42) peptides are captured by binding with the N-terminus specific antibody. The applying of C-terminus specific (1-40) antibody will bind with Abeta(l-40) only even with the Abeta(1-42) present. FIG. 6 shows that the Abeta(1-40) sample histogram 90, has the same fluorescence intensity level as the sample mixture of Abeta(1-40) and (1-42), histogram 92. Therefore, the data in FIG. 6 illustrates the reliable measurement of the concentration Abeta(1-40) only from a sample of mixture of (1-40) and (1-42) of Abeta peptides. This invention presents a method and apparatus which uses different C-terminus specific antibodies labeled with different fluorescent tags or chemiluminescence tags in combination with N-terminus specific antibody which can reliably measure concentration of each Abeta peptide variants from human tissue.