Define antigen and antibody. Name any two diagnostic kits based upon them.

Explanation

### Step-by-Step Text Solution:

**Step 1: Define Antigen**
An antigen is any substance that is recognized by the immune system as foreign. This can include pathogens like bacteria and viruses, as well as non-pathogenic substances such as pollen or dust particles. Essentially, antigens are molecules that can trigger an immune response.

**Step 2: Define Antibody**
An antibody is a type of protein produced by the immune system in response to an antigen. These proteins, also known as immunoglobulins, specifically bind to antigens to neutralize or mark them for destruction by other immune cells. There are several classes of antibodies, including Immunoglobulin A (IgA), Immunoglobulin E (IgE), Immunoglobulin M (IgM), Immunoglobulin G (IgG), and Immunoglobulin D (IgD).

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