biology

Immunoglobulin G

Immunoglobulin G is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Immunoglobulin G rather than just read about it. In short: Immunoglobulin G (IgG) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation.

Key takeaways

Immunoglobulin G belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Immunoglobulin G to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Immunoglobulin G from memory before moving on to harder problems.

Reference excerpt

Immunoglobulin G (IgG) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG antibody has two paratopes.

Function Antibodies are major components of humoral immunity. IgG is the main type of antibody found in blood and extracellular fluid, allowing it to control infection of body tissues. By binding many kinds of pathogens such as viruses, bacteria, and fungi, IgG protects the body from infection. It does this through several mechanisms:

IgG-mediated binding of pathogens causes their immobilization and binding together via agglutination; IgG coating of pathogen surfaces (known as opsonization) allows their recognition and ingestion by phagocytic immune cells, leading to the elimination of the pathogen itself; IgG activates the classical pathway of the complement system, a cascade of immune protein production that results in pathogen elimination; IgG also binds and neutralizes toxins; IgG also plays an important role in antibody-dependent cell-mediated cytotoxicity (ADCC) and intracellular antibody-mediated proteolysis, in which it binds to TRIM21 (the receptor with greatest affinity to IgG in humans) in order to direct marked virions to the proteasome in the cytosol; IgG is also associated with type II and type III hypersensitivity reactions. IgG antibodies are generated following class switching and maturation of the antibody response, thus they participate predominantly in the secondary immune response. IgG is secreted as a monomer, which is small in size, allowing it to easily diffuse into tissues. It is the only antibody isotype that has receptors to facilitate passage through the human placenta, thereby providing protection to the fetus in utero. Along with IgA secreted in the breast milk, residual IgG absorbed through the placenta provides the neonate with humoral immunity before its own immune system develops. Colostrum contains a high percentage of IgG, especially bovine colostrum. In individuals with prior immunity to a pathogen, IgG appears about 24–48 hours after antigenic stimulation. Therefore, in the first six months of life, the newborn has the same antibodies as the mother and can defend itself against all the pathogens the mother encountered in her life (even if only through vaccination) until these antibodies are degraded. This repertoire of immunoglobulins is crucial for newborns, who are very sensitive to infections, especially in the respiratory and digestive systems. IgG is also involved in regulating allergic reactions. According to Finkelman, there are two pathways of systemic anaphylaxis: antigens can cause systemic anaphylaxis in mice through classic pathway by cross-linking IgE bound to the mast cell receptor FcεRI, stimulating the release of both histamine and platelet activating factor (PAF). In the alternative pathway, antigens form complexes with IgG, which then cross-link the macrophage receptor FcγRIII, stimulating only PAF release. IgG antibodies can prevent IgE-mediated anaphylaxis by intercepting a specific antigen before it binds to mast cell–associated IgE. Consequently, IgG antibodies block systemic anaphylaxis induced by small amounts of antigen, but can mediate systemic anaphylaxis induced by larger amounts.

Structure

IgG antibodies are large globular proteins made of four peptide chains; two identical γ (gamma) heavy chains of about 50 kDa and two identical light chains of about 25 kDa. The resulting tetrameric quaternary structure, therefore, has a total molecular weight of about 150 kDa. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves, which together form a Y-like shape. Each end of the fork contains an identical antigen binding site. The various regions and domains of a typical IgG are depicted in the figure "Anatomy of an IgG." The Fc regions of IgGs bear a highly conserved N-glycosylation site at asparagine 297 in the constant region of the heavy chain. The N-glycans attached to this site are predominantly core-fucosylated biantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6-linked sialic acid residues. The N-glycan composition in IgG has been linked to several autoimmune, infectious and metabolic diseases.

Subclasses There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Immunoglobulin G

Start with the simplest possible case. Write down what Immunoglobulin G claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Immunoglobulin G before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Immunoglobulin G ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Immunoglobulin G

In research: Immunoglobulin G appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Immunoglobulin G in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Immunoglobulin G is common in secondary-school and first-year university syllabi. It links to neighbouring topics Antibodies, Glycoproteins, so understanding it makes those chapters shorter.
In everyday life: Look for Immunoglobulin G outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Immunoglobulin G in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Immunoglobulin G means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Immunoglobulin G out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Immunoglobulin G in simple terms?

Immunoglobulin G (IgG) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation.

Why does Immunoglobulin G matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Immunoglobulin G?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Immunoglobulin G.