7.1 Antibodies: Structure, Classes, Monoclonal versus Polyclonal

Antibodies (immunoglobulins) are the reagents that turn an epitope into a visible stain. Everything later in staining—direct versus indirect methods, biotinylated secondaries, polymer linkers, even dirty macrophages—starts from how IgG is built, which class you bought, and whether the bottle is a clone or a serum.

This OpenExamPrep section is independent teaching on antibody structure and format as they appear under Staining on the published QIHC topic areas. It is not an ASCP publication and does not claim Board or manufacturer approval. Detection chemistry for enzymes, chromogens, and polymers lives in Chapters 2 and 3. Here the job is the immunoglobulin itself: chains, fragments, isotype, host species, and why Fc receptors stain cells that do not contain the antigen.

Heavy chains, light chains, and the Y

A typical circulating antibody is a Y-shaped glycoprotein. Two identical heavy chains form the stem and the inner arms. Two identical light chains sit on the outer arms. Inter-chain disulfide bonds hold the tetramer together. The hinge between the arms and the stem is flexible; that flexibility is why one IgG can bind two epitopes that are not perfectly spaced.

Light chains are kappa or lambda. In a given B-cell clone both light chains match. Clinical IHC almost never asks you to pick kappa versus lambda when you buy a detection reagent. Those labels are antigens used for plasma-cell clonality, not secondary-antibody shopping rules.

Heavy chains define the isotype (class): mu for IgM, gamma for IgG, alpha for IgA, delta for IgD, epsilon for IgE. Most clinical IHC primaries and almost all labeled secondaries are IgG. IgM reagents exist—especially some older mouse monoclonals and some infectious-disease reagents—and they behave differently in storage and on the slide.

Each arm tip is a variable region (VH plus VL) that forms the paratope, the pocket that binds one epitope. The stem is the constant region, dominated by the heavy-chain Fc. Affinity is how tightly that one paratope fits one epitope. Avidity is the functional stickiness of the whole molecule: IgG with two arms, IgM with ten potential arms, and a polyclonal mixture that can occupy several epitopes on one antigen molecule.

Fab, Fc, and F(ab')2

Papain and pepsin cut IgG at different places. Those fragments are how laboratories reduce background and how some detection and blocking kits are built.

Fab versus Fc is the operational split. The Fab end is why the antibody finds the epitope. The Fc end is why a macrophage, a B cell, or a frozen lymph node can stain even when the epitope is absent: those cells display Fc receptors that grab the constant region of IgG. It is also why a goat anti-mouse secondary recognizes mouse IgG at all—the secondary is raised against mouse immunoglobulin, often with a large Fc contribution.

If a stem shows dirty histiocytes with a mouse IgG primary and an anti-mouse polymer, name Fc-receptor capture before you retiter the clone. Protein or serum block (Chapter 3) occupies those receptors. Switching to an F(ab')2 secondary or a Fab-only detection reagent removes most of the ligand the receptor wants. A biotin block does not, because the ligand is immunoglobulin, not biotin.

Some secondaries are adsorbed against human immunoglobulin so they do not also bind patient IgG already in the tissue. That adsorption is about the secondary's own specificity, not about chopping Fc. You can still have Fc-receptor dirt from the primary's Fc even when the secondary is perfectly adsorbed.

Protein A and protein G are bacterial Fc-binding proteins used in purification and in some research detection reagents. They remind you that Fc is a ligand, not dead weight. Clinical IHC secondaries are usually anti-immunoglobulin antibodies, not protein A, but the same Fc surface is involved.

IgG versus IgM

IgG is a monomer of about 150 kDa with two binding arms. It penetrates formalin-fixed paraffin-embedded (FFPE) sections reasonably, stores well, and is the default isotype for mouse and rabbit reagents. Subclasses (IgG1, IgG2a, IgG2b, and IgG3 in mouse; IgG1 through IgG4 in human and in rabbit naming) differ in hinge flexibility and in which Fc receptors they prefer. An isotype control should match the primary's subclass when the question is whether Fc or charge bound, not a random IgG from another subclass.

IgM is a pentamer in serum, about 900 kDa, with ten potential binding sites. As a primary reagent it is bulky, more prone to sticky background if washes are weak, and more sensitive to freeze-thaw and to over-dilution. Some classic mouse monoclonals from early hybridoma panels are IgM. They can work, but they are not interchangeable with an IgG clone against the same nominal antigen: affinity, epitope, and background profile differ. Do not assume an IgM primary needs the same titer or the same retrieval as an IgG.

Detection matching matters. An anti-mouse IgG polymer may report a mouse IgG1 beautifully and under-report a mouse IgM. If a stem gives a weak IgM clone after a recent polymer swap, read the vial isotype before you blame heat-induced epitope retrieval (HIER).

IgM also appears as the antigen (mu heavy chain in plasma cells) and as a direct-IF conjugate on frozen kidney or skin. Those uses are not the same as "the primary is an IgM monoclonal." Read the stem: is IgM the reagent class or the target?

Isotype as a control word

Isotype means the heavy-chain class and subclass of the immunoglobulin. Two uses collide in the laboratory.

1. Reagent isotype — the bottle is mouse IgG1, mouse IgM, or rabbit IgG. The detection reagent must see that host species and, for class-restricted secondaries, that class.
2. Isotype control — a non-immune immunoglobulin of the same species, same class, same subclass, and same working concentration as the primary, applied to a serial section. It asks whether binding was Fab-to-epitope or Fc, charge, or hydrophobic stick.

A rabbit IgG isotype control does not police a mouse IgG1 primary. A ten-fold more concentrated isotype control is a different experiment. Negative reagent controls that omit the primary test the detection chemistry (polymer, ABC, biotin). Isotype controls test the immunoglobulin format. Both can be dirty for different reasons. A clean omit-primary control with a dirty isotype control points to the primary immunoglobulin, not to streptavidin.

Monoclonal versus polyclonal

Polyclonal antibodies are a serum, or a purified immunoglobulin fraction, from an immunized animal. Many B-cell clones contribute. Many epitopes on the immunogen can be recognized. Signal can be strong because several antibodies sit on one antigen molecule—that is avidity from a mixture, not magic. The cost is lot-to-lot variability: the next bleed is not the same mixture. A new lot is a new reagent. Lot checks exist because polyclonal IHC reagents drift.

Monoclonal antibodies come from a single clone. In the classical hybridoma method, a mouse or rat plasma cell that makes the desired antibody is fused with a myeloma partner. The hybridoma is immortal and secretes one immunoglobulin: one heavy-chain class, one light chain, one paratope. Clone names are the identity. Changing the clone is not a lot tweak; it is a new assay.

Rabbit monoclonal antibodies are monoclonals produced in rabbit systems (rabbit hybridoma or recombinant rabbit IgG). They combine monoclonal reproducibility with the rabbit immune response, which often yields high-affinity reagents against epitopes that were weak in mouse. Detection still must be anti-rabbit, not anti-mouse. A universal polymer covers both. A mouse-only polymer on a rabbit monoclonal primary yields a blank slide that looks like failed retrieval.

Lot-to-lot polyclonal variability is the exam-ready phrase. A polyclonal cytokeratin that worked last year can weaken or dirty up when the vendor ships a new immunization pool. Lock the lot in the procedure, perform the new-lot check the laboratory's validation SOP requires, and do not silently dilute more without documenting a new working titer.

Mouse monoclonals have a quieter lot story: the clone is the same, but protein concentration, buffer, and carrier protein can change between fills. A concentration change is still a reagent change. Recombinant lots should be more consistent than ascites, but you still verify the fill you received.

Host species: mouse versus rabbit

Mouse primaries dominate older clinical menus because hybridoma technology started there. Rabbit primaries, polyclonal or monoclonal, dominate many newer nuclear and difficult epitopes. Neither species is more specific as a law. Specificity is the clone or the serum plus the validation, not the host.

What is a law:

• Detection species must match the primary host (Chapter 3 linker matching).
• Do not block with serum from the primary's species when the secondary is anti-that-species (Chapter 3 protein block).
• Mouse primary on mouse tissue is a mouse-on-mouse immunoglobulin problem, not a biotin problem.
• Human diagnostic FFPE with a mouse or rabbit primary is the usual clinical pairing and does not have that species overlap.

Goat, rat, and chicken primaries appear in research and in some multiplex kits. The same matching rule applies. A universal anti-mouse/anti-rabbit polymer will not see a goat primary.

Why Fc receptors matter on a staining day

Fc receptors (Fc-gamma receptors on macrophages, monocytes, dendritic cells, B cells, NK cells, and some activated epithelia) bind the Fc of IgG. Frozen lymphoid tissue is the worst case. FFPE plus HIER reduces but does not always erase the trap.

• Dirty cytoplasm in histiocytes and germinal centers with a clean expected-pattern control elsewhere → think Fc, not "the tumor is positive."
• Isotype control equally dirty in the same cells → not antigen.
• F(ab')2 or Fab detection, secondary-species serum, or a dedicated Fc block → immunologic fixes.
• Switching from ABC to polymer does not remove Fc binding; polymer still displays immunoglobulin.

The Fab binds the epitope. The Fc binds trouble. QIHC staining items that mention macrophages, frozen nodes, or nonspecific cytoplasmic staining in inflammatory cells are often Fc-receptor items, not chromogen items.

In practice

A new rabbit monoclonal arrives to replace a rabbit polyclonal against the same protein. Detection stays anti-rabbit. You still treat it as a new clone: one epitope instead of many, a new titer, a new retrieval check. You do not inherit the polyclonal's lot-to-lot folklore, and you do not keep the old polyclonal's working dilution because both bottles say rabbit.

A mouse IgM monoclonal looks weak with an anti-mouse IgG polymer. Before you blame retrieval, read the isotype on the vial. A frozen node stained with whole-molecule mouse IgG is smeared on macrophages; the same primary used with goat serum block or with an F(ab')2 secondary is clean. That is Fc-receptor biology, which is why this chapter starts with antibody anatomy rather than with DAB.