Antibodies
If B cells are the factories, then Antibodies are the precision-engineered products coming off the assembly line. Also known as Immunoglobulins (Ig), antibodies are specialised Y-shaped proteins produced by plasma cells (activated B cells) in response to foreign molecules called antigens — which include bacteria, viruses, toxins, and other harmful substances.
Antibodies work by binding specifically to an antigen — like a key fitting into a lock — thereby neutralising its harmful effects and marking it for destruction by other immune cells.
There are five major classes of antibodies (immunoglobulins), each with a unique location in the body and a specific function. A simple mnemonic to remember them: ADEGM — or think of them as five different specialised agents in the immune intelligence network.
Types of Antibodies (Immunoglobulins) — Overview
| Antibody (Ig) | Found In | Key Function / Significance |
| IgA (2nd most abundant) | Mucosal secretions — saliva, tears, breast milk, colostrum | Protects mucosal surfaces (respiratory & gastrointestinal tracts) from pathogen entry; provides passive immunity to newborns via breast milk |
| IgD (least understood) | Surface of B cells (membrane-bound) | Supports B cell maturation and activation; exact function still under research |
| IgE (smallest quantity) | Skin, lungs, mucous membranes; mast cells & basophils | Triggers allergic reactions (releases histamine); also defends against parasitic infections; elevated IgE = sign of allergy/parasitism |
| IgG (most abundant ~70–75%) | Blood and extracellular fluids (most widely distributed) | Long-term immunity after infection/vaccination; ONLY antibody to cross the placenta — provides passive immunity to fetus |
| IgM (first responder) | Blood and lymph system | First antibody produced in any new infection; critical for initial immune defence; pentameric structure gives it high antigen-binding capacity |
IgA — Immunoglobulin A
IgA is the guardian of mucosal surfaces and the second most abundant antibody in the body. Found in secretions like saliva, tears, and breast milk (especially colostrum — the first milk produced after birth), IgA’s primary role is to protect the mucous membranes of the respiratory and gastrointestinal tracts — the two major entry routes for pathogens — by preventing them from adhering to and infecting the tissue. Breast milk rich in IgA provides crucial passive immunity to newborns who haven’t yet developed their own adaptive immunity.
IgD — Immunoglobulin D
IgD is found on the surface of B cells, where it functions as a signalling receptor during B cell development. Its exact physiological function remains the least understood among all immunoglobulins.
However, it is believed to play an important role in B cell maturation and activation — essentially helping young B cells recognise antigens and become immunologically active. Remember: IgD sits on the B cell surface and its function is still not fully clear.
IgE — Immunoglobulin E
IgE is the antibody most closely associated with allergic reactions and anti-parasitic defence. Normally found in very small amounts in the blood, IgE is produced in response to allergens (such as pollen, pet dander, dust mites, or certain foods).
When IgE binds to an allergen, it triggers mast cells and basophils to release histamine and other inflammatory chemicals, causing familiar allergic symptoms: itching, swelling, runny nose, and breathing difficulty.
In severe cases, this IgE-mediated reaction can cause anaphylaxis — a rapid, life-threatening allergic shock that requires immediate epinephrine (adrenaline) administration. Elevated levels of IgE in a blood test are a reliable clinical indicator of either an allergic condition or a parasitic infection.
IgE also plays a protective role against parasitic infections such as those caused by helminths (worms).
IgG — Immunoglobulin G
IgG is the most abundant antibody in the human body, accounting for approximately 70–75% of all immunoglobulins. Found mainly in the blood and extracellular fluids, IgG provides long-term immunity following infection or vaccination — it is the antibody your body builds in large quantities after the initial IgM-dominated response subsides.
IgG’s most remarkable and important property is that it is the only antibody that can cross the placenta, thereby transferring maternal immunity to the developing fetus. This is the primary mechanism of natural passive immunity in newborns.
IgG is also the antibody most widely used in therapeutic and diagnostic applications (e.g., monoclonal antibodies in cancer therapy).
IgM — Immunoglobulin M
IgM is the first responder of the antibody world — literally the first antibody the immune system produces whenever it encounters a new infection for the very first time. Found in the blood and lymph system, IgM has a distinctive pentameric structure (five antibody units joined together like a star), giving it an exceptionally high capacity to bind multiple antigens simultaneously.
In clinical diagnostics, the presence of IgM antibodies in the blood indicates an acute (recent or current) infection — because IgM is only produced in the early phase of an immune response before IgG takes over.
This makes IgM testing a valuable tool for detecting fresh infections, including dengue, malaria, and COVID-19, among others.
Quick Recall
| Class | Key Word | Remember As |
| IgA | Mucosal / Secretions | A for ‘Areas of entry’ — guards all entry-point surfaces; in breast milk |
| IgD | B cell surface / Activation | D for ‘Developmental signal’ — helps B cells mature and recognise antigen |
| IgE | Allergy / Parasites | E for ‘Emergency allergy signal’ — causes histamine release; elevated in allergies |
| IgG | Most abundant / Placenta | G for ‘Guardian long-term’ — crosses placenta; most common; long-lasting |
| IgM | First produced / Acute infection | M for ‘Must respond first’ — first antibody on scene; pentameric structure |