Immunoglobulins: Structure and Function
The correct answer is E: 1 and 3, as immunoglobulins are produced by plasma cells and the antigen-antibody combining sites are located on the Fab fragment of the immunoglobulin.
Detailed Analysis of Each Statement
Statement 1: Immunoglobulins are produced by plasma cells
- TRUE. Plasma cells are the primary producers of immunoglobulins (antibodies) in the body. This is well-established in immunology literature 1. Plasma cells develop from B lymphocytes that have been activated by antigen exposure and subsequently differentiate into antibody-secreting cells 2.
Statement 2: There are six classes of immunoglobulins all produced by plasma cells
- FALSE. There are five main classes (isotypes) of immunoglobulins: IgG, IgM, IgA, IgD, and IgE 1. IgG can be further divided into four subclasses (IgG1, IgG2, IgG3, and IgG4), and IgA can be divided into two subclasses (IgA1 and IgA2), but these are subclasses within the main isotypes, not separate classes themselves 1, 3.
Statement 3: The antigen-antibody combining sites are located on the Fab fragment of the immunoglobulin
- TRUE. The variable domains of immunoglobulins form the antigen-binding sites, which are located on the Fab (Fragment, antigen-binding) portion of the antibody molecule 1. Specifically, the three complementarity-determining regions (CDRs) of the heavy chain pair with the three CDRs of the light chain to form the antigen-binding site 1, 4.
Statement 4: An immunoglobulin molecule is composed of two different heavy chains and the same light chains
- FALSE. An immunoglobulin molecule is composed of two identical heavy chains and two identical light chains arranged in a Y-shaped structure 1. The heavy chains are identical to each other, and the light chains are identical to each other within a single antibody molecule 2, 4.
Structure and Function of Immunoglobulins
Immunoglobulins are heterodimeric proteins with a complex structure that enables their diverse functions:
Basic Structure: Each immunoglobulin consists of two identical heavy chains and two identical light chains connected by disulfide bonds 1
Functional Domains:
- Variable domains: Located in the Fab portion, responsible for antigen binding
- Constant domains: Determine effector functions like complement activation or Fc receptor binding 1
Diversity Generation:
- V-D-J gene recombination
- Junctional diversity
- Somatic hypermutation
- Combinatorial association of light and heavy chains 2
Classes and Functions:
- IgG: Main antibody in blood, can cross placenta, activates complement (especially IgG1/3)
- IgM: First antibody produced in immune response, effective at complement activation
- IgA: Predominant antibody at mucosal surfaces, exists in dimeric form in secretions
- IgE: Involved in allergic reactions and parasite defense
- IgD: Functions in B-cell development 1, 3
Clinical Significance
Understanding immunoglobulin structure and function is critical for:
Diagnostic Applications: Serology, immunohistochemistry, immunofluorescence, and other immunoassays 2
Therapeutic Uses:
Monoclonal Antibody Development: For targeted therapies in cancer, autoimmune diseases, and other conditions 2, 6
Common Pitfalls and Misconceptions
Confusing subclasses with main classes: While there are multiple subclasses of IgG and IgA, there are only five main immunoglobulin classes 1
Assuming all antibodies have identical structure: While the basic Y-shaped structure is conserved, significant variations exist between classes in terms of heavy chain structure, glycosylation patterns, and ability to form multimers 1, 3
Overlooking the importance of plasma cells: B cells are often incorrectly cited as the main producers of antibodies, but it's specifically the terminally differentiated plasma cells that secrete large amounts of antibodies 2