14. Based on the heavy chain, which one of the following antibodies has multiple subtypes?
(A) IgM
(B) IgD
(C) IgE
(D) IgG
IgG is the antibody with multiple subtypes based on its heavy chain among the options. The five antibody classes differ by heavy chain type, but only IgG features subclasses with distinct gamma heavy chain variants. This makes option (D) IgG the correct answer.
Option Analysis
IgM (Option A): IgM uses a single μ (mu) heavy chain type and lacks subclasses. It functions as the first responder in infections, forming pentamers for strong complement activation.
IgD (Option B): IgD has one δ (delta) heavy chain with no subtypes. Primarily on naive B cells, it aids early antigen recognition but remains minor in serum.
IgE (Option C): IgE relies on a single ε (epsilon) heavy chain without subtypes. It triggers allergic responses and fights parasites via mast cell binding.
IgG (Option D): IgG has four subtypes (IgG1, IgG2, IgG3, IgG4) from γ (gamma) heavy chain variations differing in hinge regions and effector functions like complement activation and Fc receptor binding.
Heavy Chain Subtype Details
Antibody classes arise from heavy chain differences: μ for IgM, δ for IgD, ε for IgE, α for IgA (with two subtypes, not listed), and γ for IgG. IgG subclasses vary in abundance (IgG1 ~60%, IgG2 ~32%, IgG3 ~4%, IgG4 ~4%), half-life, and roles—IgG1 excels against proteins, IgG2 against polysaccharides. These enable specialized immunity, such as IgG3’s potent complement fixation.
| Subclass | Heavy Chain Features | Key Functions | Serum Abundance |
|---|---|---|---|
| IgG1 | 15 hinge amino acids, 2 disulfide bonds | Protein antigens, ADCC, placental transfer | 60% |
| IgG2 | 12 hinge amino acids, 4 disulfide bonds | Polysaccharides, weak complement | 32% |
| IgG3 | 62 hinge amino acids, 11 disulfide bonds | Strong complement, allergens | 4% |
| IgG4 | 12 hinge amino acids, no complement | Chronic antigens, poor effector | 4% |
Immunology Relevance
IgG subtypes optimize responses: deficiencies in IgG2 heighten encapsulated bacteria risk. In biotech and your molecular biology focus, they guide therapeutic design like monoclonal antibodies. This question tests isotype knowledge vital for exams in genetics and immunology.
