Antigens Practice Questions

Correct answer: Hapten

Haptens are low-molecular-weight compounds that can be recognized by antibodies but cannot initiate an immune response on their own. When bound to a high-molecular-weight carrier protein, they become part of an immunogenic complex.

Correct answer: Epitope

Epitopes, or antigenic determinants, are the discrete sites on an antigen that physically interact with the paratope of an antibody or the TCR. A single complex antigen usually possesses multiple different epitopes.

Correct answer: Proteins

Proteins are highly immunogenic due to their large size, structural complexity, and chemical heterogeneity. Their ability to be processed into peptides and presented on MHC molecules also allows for strong T-cell involvement.

Correct answer: Adjuvant

Adjuvants, such as aluminum salts (alum), enhance the immunogenicity of an antigen by prolonging its persistence in the body or by activating innate immune receptors. They help achieve a stronger and longer-lasting protective immunity.

Correct answer: T-dependent antigens require T-cell help to activate B cells

T-dependent antigens (mostly proteins) require signals from helper T cells to induce B-cell isotype switching and memory. T-independent antigens can activate B cells directly but typically result in a weaker, primarily IgM-based response with little memory.

Correct answer: Superantigen

Superantigens, such as Toxic Shock Syndrome Toxin-1 (TSST-1), bypass the traditional antigen-processing pathway. By binding non-specifically to the outside of the TCR and MHC II, they activate up to 20% of the T-cell population simultaneously.

Correct answer: Alloantigens

Alloantigens are antigens present in some but not all members of the same species. They are clinically significant in blood transfusions, organ transplantation, and hemolytic disease of the newborn.

Correct answer: Are shared by phylogenetically unrelated species

Heterophile antigens are identical or closely related antigens found in different species (e.g., bacteria and humans). Antibodies produced against one may cross-react with the other, as seen in the Weil-Felix or Paul-Bunnell tests.

Correct answer: High degree of foreignness

Foreignness is a prerequisite for immunogenicity; the more distant the evolutionary relationship between the antigen source and the host, the more likely the substance is to be recognized as non-self and elicit a response.

Correct answer: MHC Class I molecules

Endogenous antigens, such as viral proteins synthesized inside an infected cell, are degraded by proteasomes. The resulting peptides are transported to the ER and loaded onto MHC Class I molecules for presentation to CD8+ cytotoxic T cells.

Correct answer: Conformational epitope

Conformational (discontinuous) epitopes are composed of amino acids that are far apart in the primary sequence but brought together by protein folding. If the protein is denatured or unfolded, the antibody can no longer recognize the epitope.

Correct answer: Pneumococcal capsular polysaccharide

TI-2 antigens are highly repetitive, multivalent structures like polysaccharides. They cross-link multiple B-cell receptors simultaneously, providing a strong enough signal for B-cell activation without T-cell help, though they are poorly immunogenic in infants.

Correct answer: The antibody response against a hapten-carrier conjugate

While B cells recognize the hapten, the T cells must recognize peptides from the carrier protein presented on MHC II. Without the T-cell help generated by the carrier, the B cell cannot undergo class switching or produce high titers of anti-hapten antibodies.

Correct answer: Lens protein of the eye

Sequestered antigens are hidden from the immune system during the development of self-tolerance. If tissues like the lens of the eye or sperm are damaged and release these antigens into the blood, the immune system may treat them as foreign.

Correct answer: Organ-specific antigens

Organ-specific antigens are restricted to a particular organ or tissue (e.g., thyroglobulin in the thyroid). They are often the targets in organ-specific autoimmune diseases.

Correct answer: MHC Class II

Exogenous antigens originate outside the cell (e.g., extracellular bacteria). They are internalized into endosomes/lysosomes where they are degraded into peptides and loaded onto MHC Class II molecules for presentation to CD4+ helper T cells.

Correct answer: It occurs when two different antigens share the same or very similar epitopes

Cross-reactivity occurs when an antibody raised against one antigen binds to a different antigen because they share identical or structurally similar epitopes. This can lead to diagnostic errors or autoimmune conditions like rheumatic fever.

Correct answer: The total number of epitopes on the antigen surface

Antigen valency is the number of determinants (epitopes) per molecule. Most natural antigens are multivalent, meaning they can bind to several antibody molecules simultaneously, which facilitates the formation of large immune complexes.

Correct answer: Proteins

Unlike the ABO system (which involves carbohydrate antigens), the Rh (Rhesus) antigens are transmembrane proteins. This makes Rh antigens highly immunogenic, particularly the D antigen.

Correct answer: Antigens present on both normal and tumor cells but overexpressed on tumors

Tumor-Associated Antigens (TAAs) are present on normal cells but show abnormal expression levels or patterns on tumor cells (e.g., CEA or PSA). They differ from Tumor-Specific Antigens (TSAs), which are unique to malignant cells.