Source: PCB3023L Assigned Reading, Protein Synthesis & Signaling
Difficulty: Intermediate | Prerequisites: Cell signalling basics (see companion notes), basic immunology concepts helpful but not required
This section bridges theory and lab practice. The cell signalling notes explain what happens inside a cell when a signal arrives. These notes explain how researchers observe and measure those events using antibodies as molecular probes. If you understand what EGFR does but have no idea how a lab detects whether it is active, this is the missing piece. Antibody-based techniques (Western blotting, immunofluorescence) are standard tools in cell biology research and are likely to appear in both practical and written assessments.
Antibodies are proteins made by the immune system that bind specific targets with high precision. Researchers use them as tools to detect, quantify, and localise signalling proteins in the lab. In the PCB3023L project, anti-EGFR and phospho-AKT antibodies are used in Western blots and immunofluorescence to track EGFR activation and downstream PI3K/AKT signalling.
Antibody (immunoglobulin)
A Y-shaped protein produced by B cells that binds a specific molecular target (antigen) with high affinity. Used in research as a detection tool. Think of it as a highly specific molecular clamp that grabs only one target.
Antigen
Any substance that triggers an immune response and can be bound by an antibody. In research, the antigen is typically the protein or peptide fragment you want to detect.
Epitope
The specific region on an antigen that an antibody recognises and binds. A single protein can have many different epitopes. In simple terms, the exact "patch" on a protein that the antibody locks onto.
Polyclonal antibodies
A mixture of antibodies produced by many different B cells, recognising multiple epitopes on the same antigen. Generated by immunising an animal and collecting serum. Broader coverage, but less consistent between batches.
Monoclonal antibodies
Antibodies produced from a single cloned B cell line, all recognising exactly the same epitope. Higher specificity and batch-to-batch consistency.
Recombinant antibodies
Antibodies engineered in vitro (outside a living animal), offering precise control over binding properties.
Western blotting (immunoblotting)
A laboratory technique that separates proteins by size using gel electrophoresis, transfers them to a membrane, and detects specific proteins using antibodies. Used to quantify protein expression and detect post-translational modifications like phosphorylation.
Immunofluorescence
A microscopy technique that uses fluorescently labelled antibodies to visualise the spatial distribution of a protein within cells.
Phospho-specific antibody
An antibody engineered to recognise a protein only when a particular residue is phosphorylated. Essential for detecting whether a signalling protein is in its active state.
Anti-EGFR XP Rabbit antibody
The specific antibody used in the PCB3023L project to detect total EGFR protein levels regardless of activation state.
Phospho-AKT antibody
The antibody used to detect AKT only when it is phosphorylated (active), serving as a readout for PI3K/AKT pathway activation downstream of EGFR.
Antibodies are immunoglobulin proteins naturally produced by B cells of the immune system. Their job in the body is to recognise and bind foreign substances (antigens) such as pathogens or toxins. Researchers exploit this binding specificity to detect proteins of interest in the lab.
How research antibodies are made:
Polyclonal antibodies: An animal (commonly a rabbit, mouse, or goat) is injected with a purified protein or peptide fragment. The animal's immune system generates a diverse set of antibodies against multiple epitopes on that protein. Serum is collected and the antibodies are purified. These are quick to produce and recognise the target from multiple angles, but vary from batch to batch.
Monoclonal antibodies: A single B cell producing an antibody against one specific epitope is isolated and cloned (typically fused with a myeloma cell to create a hybridoma). The result is a population of identical antibodies, all binding the same epitope. Higher specificity and reproducibility, but more labour-intensive to produce.
Recombinant antibodies: Engineered in vitro using molecular biology techniques, bypassing the need for animal immunisation entirely. Offer the greatest control over binding properties and are increasingly common in modern research.
The choice between polyclonal, monoclonal, and recombinant depends on the experiment. Monoclonal antibodies are preferred when you need to detect a single, specific modification (such as phosphorylation at one particular residue). Polyclonal antibodies are useful when you want robust detection of a protein regardless of its modification state.
The PCB3023L project uses antibodies to study EGFR signalling in two complementary ways:
Antibody 1: Anti-EGFR XP Rabbit
Detects total EGFR protein, regardless of whether the receptor is active or inactive.
Used to track how EGFR protein levels change over time after ligand stimulation.
Tells you "how much EGFR is present," not "how much EGFR is active."
Antibody 2: Phospho-AKT
Detects AKT only when it is phosphorylated (i.e. activated).
Serves as a downstream readout: if phospho-AKT levels increase after EGF stimulation, it confirms that the PI3K/AKT pathway is being activated by EGFR.
Tells you "the signalling cascade is firing," not just that the receptor is present.
Techniques used:
Western blotting separates proteins by molecular weight on a gel, transfers them to a membrane, and probes with the antibody. Produces bands whose intensity is proportional to the amount of target protein. Good for quantification and for comparing conditions (e.g. stimulated vs. unstimulated, different time points).
Immunofluorescence uses fluorescently tagged antibodies and microscopy to visualise where the protein is located within the cell. Good for answering spatial questions: is EGFR on the membrane or has it been internalised? Is phospho-AKT concentrated in the cytoplasm or nucleus?
Using both antibodies together gives a more complete picture: anti-EGFR shows total receptor levels, and phospho-AKT confirms that downstream signalling is active. This is how the project connects receptor activation to functional cellular outcomes like proliferation and survival.
Antibody-based detection is everywhere in biomedicine. Pregnancy tests use monoclonal antibodies to detect hCG. COVID rapid tests use antibodies against viral proteins. In clinical labs, Western blots confirm HIV diagnosis. Cancer diagnostics use immunohistochemistry (a close relative of immunofluorescence) to classify tumours by their receptor status, for example HER2 status in breast cancer, which determines treatment options.
Students often confuse polyclonal and monoclonal antibodies. Polyclonal recognise multiple epitopes on the same antigen (a mixture of antibodies). Monoclonal recognise one specific epitope (all identical antibodies from one B cell clone).
Students sometimes think that detecting total protein (anti-EGFR) tells you about activation. It does not. You need a phospho-specific antibody to determine whether the protein is in its active, phosphorylated state.
Students sometimes assume Western blotting and immunofluorescence give the same information. They complement each other: Western blots quantify protein levels, immunofluorescence shows protein location within the cell.
⚠️ Know the difference between polyclonal, monoclonal, and recombinant antibodies, including how each is produced and when each is preferred.
⚠️ Be able to explain why the project uses two different antibodies (anti-EGFR for total protein, phospho-AKT for pathway activation) and what each one tells you.
⚠️ Understand the difference between Western blotting (quantification, size-based separation) and immunofluorescence (spatial localisation).
⚠️ Be able to explain what a phospho-specific antibody detects and why this is important for studying signalling.
True or False: Polyclonal antibodies bind a single epitope on an antigen.
Fill in the blank: A phospho-specific antibody detects a protein only when it is ______.
True or False: Western blotting tells you where a protein is located inside the cell.
Fill in the blank: In the EGFR project, the ______ antibody serves as a readout for PI3K/AKT pathway activation.
True or False: Monoclonal antibodies are produced by immunising an animal and collecting serum.
(Answers: 1. False, polyclonal antibodies recognise multiple epitopes. 2. Phosphorylated. 3. False, that is immunofluorescence; Western blotting quantifies protein levels. 4. Phospho-AKT. 5. False, monoclonal antibodies are produced from a single cloned B cell line (hybridoma).)
Q: Explain the difference between polyclonal and monoclonal antibodies, including how each is produced.
A: Polyclonal antibodies are produced by immunising an animal with an antigen and collecting serum. They are a mixture of antibodies from many B cells, recognising multiple epitopes. Monoclonal antibodies come from a single B cell clone (hybridoma), so all copies bind the same epitope. Monoclonal are more specific and reproducible; polyclonal are quicker to produce and detect the target from multiple angles.
Q: In the PCB3023L project, why are two different antibodies used to study EGFR signalling?
A: The anti-EGFR antibody detects total EGFR protein levels regardless of activation state, showing how much receptor is present. The phospho-AKT antibody detects activated (phosphorylated) AKT downstream, confirming that the signalling cascade from EGFR through PI3K/AKT is functional. Together they distinguish between receptor presence and pathway activation.
Q: Compare what Western blotting and immunofluorescence each tell you about a signalling protein.
A: Western blotting separates proteins by size and quantifies how much of a target protein is present under different conditions. Immunofluorescence uses fluorescent antibodies and microscopy to show where a protein is located within the cell. Western blotting answers "how much," immunofluorescence answers "where."
Q: What is the advantage of a phospho-specific antibody over an antibody that detects total protein?
A: A total-protein antibody tells you the protein is present but says nothing about whether it is active. A phospho-specific antibody detects the protein only when a key residue is phosphorylated, which typically corresponds to the active state. This lets researchers determine whether a signalling pathway is turned on, not just whether its components exist.
This material connects directly to the cell signalling notes: the antibodies discussed here are the tools used to detect the signalling events (EGFR activation, AKT phosphorylation) described there. You cannot fully understand one without the other.
Antibody generation ties back to immunology (B cell activation, clonal selection) and to protein structure (epitopes, antigen-antibody binding depends on 3D shape).
Western blotting and immunofluorescence will likely reappear throughout the course and in lab practicals, so understanding their principles here pays dividends later.
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