BP104T • Unit 4Topic 3 of 4⏱ 5 Min Read

3. Immunoglobulin Architecture & The 5 Classes of Antibodies

💡 Core Concept & Clinical Context:

Antibodies (Immunoglobulins - Igs) are Y-shaped glycoprotein molecules synthesized by plasma cells. An individual antibody consists of four polypeptide chains: two identical Heavy (H) chains and two identical Light (L) chains joined by interchain disulfide bonds. The antibody molecule is functionally divided into the Fab region (Fragment antigen-binding, containing the variable $V_H$ and $V_L$ domains that form the antigen-combining paratope) and the Fc region (Fragment crystallizable, which binds to immune cell Fc receptors and activates the classical complement cascade). Human immunoglobulins are classified into 5 distinct isotypes based on heavy chain structure: IgG, IgM, IgA, IgE, and IgD.

Clinical Scenarios & Pharmacopoeial Examples:

1Distinctive Biological Roles of the 5 Immunoglobulin Classes

Structural and clinical properties of IgG, IgM, IgA, IgE, and IgD.

Step-by-Step Breakdown:

•• IgG (Monomer, ~75-80% of serum Igs):

•- Most abundant circulating antibody.

•- ONLY antibody class that crosses human placenta, providing passive immunity to fetus and neonate.

•- Mediates opsonization, complement fixation, and secondary immune response.

•• IgM (Pentamer joined by J-chain, ~10%):

•- Largest antibody (Mol. Wt. ~900 kDa); 10 theoretical antigen-binding sites.

•- First antibody synthesized in primary immune response; potent complement activator.

•• IgA (Dimer with J-chain & Secretory Component, ~15%):

•- Predominant antibody in mucosal secretions (Saliva, Tears, Colostrum, Breast milk, GI & Respiratory mucus).

•- Prevents adherence and colonization of pathogens on mucosal surfaces.

•• IgE (Monomer, < 0.01%):

•- Binds high-affinity Fc-epsilon-RI receptors on Mast Cells and Basophils.

•- Triggers degranulation (histamine release) in Type I Immediate Allergic reactions and defense against parasitic helminths.

•• IgD (Monomer, < 1%):

•- Co-expressed with IgM on naive B-cell surfaces; functions as an antigen receptor for B-cell activation.

Comparison & Reference Table:

Ig Class	Physical Structure	Serum Half-Life	Crosses Placenta?	Primary Clinical Function
IgG	Monomer	~21 - 23 days	YES (Only class)	Long-term immunity, opsonization, neutralization of toxins
IgM	Pentamer with J-chain	~5 days	NO	Primary acute response, agglutination, complement fixation
IgA	Dimer + Secretory piece	~6 days	NO	Mucosal mucosal surface protection, present in colostrum
IgE	Monomer	~2 days	NO	Allergic immediate hypersensitivity, anti-parasitic defense
IgD	Monomer	~3 days	NO	B-lymphocyte surface antigen recognition receptor

📋Key Takeaways & Exam Highlights

IgG: Crosses placenta; dominant in secondary booster immune responses.
IgM: Pentameric structure; first antibody produced during acute infection.
IgA: Secretory dimer; protects mucous membranes and infant gut via breast milk.
IgE: Reaginic antibody responsible for anaphylaxis and asthma.

🇮🇳 हिन्दी व्याख्या (3. Immunoglobulin Architecture & The 5 Classes of Antibodies)

एंटीबॉडी 5 प्रकार की होती हैं (GAMED): IgG सबसे ज्यादा होती है और प्लेसेंटा पार कर सकती है, IgM सबसे पहले बनती है, IgA मां के दूध और लार में होती है, और IgE एलर्जी तथा अस्थमा में हिस्टामाइन छोड़ती है।