Which of the following statements is true?
a. An antibody has one antigen-binding site
b. A lymphocyte has receptors for a single antigen
c. Every antigen has a single epitope
d. A liver or muscle cell makes two classes of MHC molecule

Verified step by step guidance

1. Step 1: Understand the key terms in the question. An antibody is a protein produced by B cells that binds specifically to antigens. Antigens are molecules that elicit an immune response, and epitopes are the specific parts of an antigen that antibodies or receptors bind to. MHC (Major Histocompatibility Complex) molecules are proteins on the surface of cells that present antigens to T cells.
2. Step 2: Evaluate statement (A): 'An antibody has one antigen-binding site.' Recall that antibodies are Y-shaped molecules with two identical antigen-binding sites, one on each arm of the Y. This makes statement (A) false.
3. Step 3: Evaluate statement (B): 'A lymphocyte has receptors for a single antigen.' Lymphocytes (B cells and T cells) are highly specific and typically have receptors that recognize only one specific antigen. This makes statement (B) true.
4. Step 4: Evaluate statement (C): 'Every antigen has a single epitope.' Antigens can have multiple epitopes, allowing different antibodies or receptors to bind to different parts of the antigen. This makes statement (C) false.
5. Step 5: Evaluate statement (D): 'A liver or muscle cell makes two classes of MHC molecule.' Liver and muscle cells are not immune cells and typically express only MHC class I molecules, not both classes. This makes statement (D) false.

Key Concepts

Antibodies and Antigen-Binding Sites

Antibodies are proteins produced by B cells that specifically bind to antigens, which are foreign substances that trigger an immune response. Each antibody typically has two identical antigen-binding sites, allowing it to bind to two identical epitopes on an antigen, enhancing its ability to neutralize pathogens.

Lymphocyte Receptors

Lymphocytes, including B cells and T cells, have specific receptors that recognize and bind to particular antigens. Each lymphocyte is programmed to recognize a single antigen, which allows for a highly specific immune response. This specificity is crucial for the adaptive immune system's ability to target diverse pathogens.

Epitopes and Antigens

An epitope is the specific part of an antigen that is recognized by the immune system, particularly by antibodies or T cell receptors. Antigens can have multiple epitopes, meaning they can be recognized by different antibodies or T cells, which allows for a robust immune response against a single pathogen.