Which of the following statements is not true?
a. An antibody has more than one antigen-binding site.
b. An antigen can have different epitopes.
c. A lymphocyte has receptors for multiple and different antigens.
d. A bacterium has more than one antigen.

Verified step by step guidance

1. Step 1: Understand the key terms in the question. An antibody is a protein produced by the immune system that binds to specific antigens. Antigens are molecules capable of inducing an immune response, and epitopes are specific parts of an antigen that antibodies or receptors bind to. Lymphocytes are immune cells (e.g., B cells and T cells) that have receptors to recognize antigens.
2. Step 2: Analyze statement (a): 'An antibody has more than one antigen-binding site.' Recall that antibodies are Y-shaped molecules with two identical antigen-binding sites, meaning this statement is true.
3. Step 3: Analyze statement (b): 'An antigen can have different epitopes.' Antigens often have multiple epitopes, allowing different antibodies or receptors to bind to different parts of the same antigen. This statement is true.
4. Step 4: Analyze statement (c): 'A lymphocyte has receptors for multiple and different antigens.' Lymphocytes are highly specific; each lymphocyte has receptors that recognize only one specific antigen. This statement is not true.
5. Step 5: Analyze statement (d): 'A bacterium has more than one antigen.' Bacteria are complex organisms with many molecules on their surface, each potentially acting as an antigen. This statement is true. Therefore, the incorrect statement is (c).

Key Concepts

Antibodies and Antigen-Binding Sites

Antibodies are proteins produced by B cells that recognize and bind to specific antigens. Each antibody typically has two identical antigen-binding sites, allowing it to bind to two identical epitopes on an antigen. This multivalency enhances the ability of antibodies to cross-link antigens, facilitating their neutralization and removal from the body.

Antigens and Epitopes

Antigens are substances that can provoke an immune response, often found on the surface of pathogens like bacteria and viruses. An antigen can have multiple distinct regions known as epitopes, which are the specific parts recognized by antibodies. This diversity allows the immune system to target various aspects of a pathogen effectively.

Lymphocyte Receptors

Lymphocytes, including B cells and T cells, are crucial components of the adaptive immune system. Each lymphocyte has receptors that are specific to a particular antigen, but a single lymphocyte typically expresses receptors for only one type of antigen. This specificity is essential for the targeted immune response, as it ensures that each lymphocyte responds to a unique pathogen.