science

Lectin pathway

Lectin pathway is a science 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 Lectin pathway rather than just read about it. In short: The lectin pathway or MBL pathway is a type of cascade reaction in the complement system, similar in structure to the classical complement pathway, in that, after activation, it proceeds through the action of C4 and C2 to produce activated complement proteins further down the cascade. In contrast to the classical complement pathway, the lectin pathway does not recognize an antibody bound to its target.

Key takeaways

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

Reference excerpt

The lectin pathway or MBL pathway is a type of cascade reaction in the complement system, similar in structure to the classical complement pathway, in that, after activation, it proceeds through the action of C4 and C2 to produce activated complement proteins further down the cascade. In contrast to the classical complement pathway, the lectin pathway does not recognize an antibody bound to its target. The lectin pathway starts with mannose-binding lectin (MBL) or ficolin binding to certain sugars. In this pathway, mannose-binding lectin binds to mannose, glucose, or other sugars with 3- and 4-OH groups placed in the equatorial plane, in terminal positions on carbohydrate or glycoprotein components of microorganisms including bacteria such as Salmonella, Listeria, and Neisseria strains. Fungal pathogens such as Candida albicans and Cryptococcus neoformans as well as some viruses such as HIV-1 and Respiratory syncytial virus (RSV) are bound by MBL. Mannan-binding lectin, also called mannose-binding protein or mannose-binding-lectin (MBL), is a protein belonging to the collectin family that is produced by the liver and can initiate the complement cascade by binding to pathogen surfaces.

MBL MBL forms oligomers of subunits, which are trimers (such that 6- and 18-subunit oligomers correspond to a dimer and a hexamer, respectively). Multimers of MBL form a complex with MASP1 (Mannose-binding lectin-Associated Serine Protease), MASP2 and MASP3, that are protease zymogens. The MASPs are very similar to C1r and C1s molecules of the classical complement pathway. When the carbohydrate-recognising heads of MBL bind to specifically arranged mannose residues on the surface of a pathogen, MASP-1 and MASP-2 are activated to cleave complement components C4 and C2 into C4a, C4b, C2a, and C2b. In addition, two smaller MBL-associated proteins (MAps) are found in complex with MBL. MBL-associated protein of 19 kDa (MAp19) and MBL-associated protein of 44 kDa (Map44). MASP-1, MASP-3 and MAp44 are alternative splice products of the MASP1 gene, while MASP-2 and MAp19 are alternative splice products of the MASP-2 gene. MAp44 has been suggested to act as a competitive inhibitor of lectin pathway activation, by displacing MASP-2 from MBL, hence preventing cleavage of C4 and C2

C3 convertase C4b tends to bind to bacterial cell membranes. If it is not then inactivated, it will combine with C2a to form the classical C3 convertase (C4bC2a) on the surface of the pathogen, as opposed to the alternative C3 convertase (C3bBb) involved in the alternative pathway. C4a and C2b act as potent cytokines, with C4a causing degranulation of mast cells and basophils and C2b acting to increase vascular permeability. Historically, the larger fragment of C2 was called C2a but some publications now refer to it as C2b in keeping with the convention of assigning 'b' to the larger fragment.

Clinical significance Mannose-binding Lectin deficiency - These individuals are prone to recurrent infections, including infections of the upper respiratory tract and other body systems. People with this condition may also contract more serious infections such as pneumonia and meningitis. Depending on the type of infection, the symptoms caused by the infections vary in frequency and severity. Although the clinical significance of MBL-Deficiency is debated. Infants and young children with mannose-binding lectin deficiency seem to be more susceptible to infections, but adults can also develop recurrent infections. In addition, affected individuals undergoing chemotherapy or taking drugs that suppress the immune system are especially prone to infections.

See also Classical complement pathway Alternative complement pathway Mannan-binding lectin

References

External links nih.gov Ali, Youssif M.; Lynch, Nicholas J.; Haleem, Kashif S.; Fujita, Teizo; Endo, Yuichi; Hansen, Soren; Holmskov, Uffe; Takahashi, Kazue; Stahl, Gregory L.; Dudler, Thomas; Girija, Umakhanth V.; Wallis, Russell; Kadioglu, Aras; Stover, Cordula M.; Andrew, Peter W.; Schwaeble, Wilhelm J. (5 July 2012). "The Lectin Pathway of Complement Activation Is a Critical Component of the Innate Immune Response to Pneumococcal Infection". PLOS Pathogens. 8 (7) e1002793. doi:10.1371/journal.ppat.1002793. PMC 3390405. PMID 22792067.

Illustrations

Worked examples

Example 1 — a first encounter with Lectin pathway

Start with the simplest possible case. Write down what Lectin pathway claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Lectin pathway 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 Lectin pathway 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 Lectin pathway

In research: Lectin pathway appears in science 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 Lectin pathway 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: Lectin pathway is common in secondary-school and first-year university syllabi. It links to neighbouring topics Complement system, so understanding it makes those chapters shorter.
In everyday life: Look for Lectin pathway 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 Lectin pathway in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lectin pathway 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 Lectin pathway out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lectin pathway in simple terms?

Why does Lectin pathway matter?

Because it connects several science 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 Lectin pathway?

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 Lectin pathway.