Antigen and Antibody

Antigen

Definition: binds specifically to antibody or TCR. Immunogen: antigen that can ALSO trigger immune response by itself. All immunogens = antigens, not all antigens = immunogens.

Immunogenicity factors:

• Foreignness — most important, self vs non-self degree
• Size — larger (>10kDa) = more immunogenic
• Chemical complexity — Protein (most immunogenic) > Polysaccharide > Lipid/Nucleic acid (weak alone, need carrier)
• Physical form — particulate/aggregated > soluble
• Susceptibility to processing — must be degradable + MHC-presentable for T-cell-dependent response

Epitope and Hapten

Epitope (antigenic determinant): specific small region recognized by Ab/TCR. One antigen = multiple epitopes. B cell epitope: surface, conformational (3D dependent). T cell epitope: linear peptide (processed, unfolded).

Hapten: small, antigenic but NOT immunogenic alone. Needs carrier (protein) coupling → hapten-carrier complex → immunogenic. Examples: penicillin allergy, nickel dermatitis (drug/metal + host protein).

Classification

By origin: Exogenous (outside → endocytic pathway → MHC-II) vs Endogenous (inside host cell — viral/tumor protein → cytosolic pathway → MHC-I).

By chemical nature: Protein, Polysaccharide, Lipid, Nucleic acid.

By T-cell dependence:

• Thymus-dependent (TD) — most protein antigens, need T help, memory + class switch
• Thymus-independent (TI) — bacterial polysaccharide/LPS, direct B cell activation (BCR cross-linking), mainly IgM, weaker memory

By relation to host:

• Autoantigen — self, normally tolerated
• Alloantigen — same species, genetically different (transplant rejection, blood group)
• Heteroantigen — different species
• Heterophile antigen — shared across unrelated species/genera, cross-reacts (Weil-Felix, Paul-Bunnell test basis)

Other: Superantigens (microbial, e.g. staph toxins — bypass processing, bridge TCR Vβ directly to MHC-II, massive nonspecific T cell activation). Tumor antigens — see Transplant/Tumor Immunology topic.

Antibody (Immunoglobulin)

Structure: Y-shaped, 4 chains — 2 heavy + 2 light, disulfide-linked. Variable region (N-terminus, antigen-binding, differs per Ab) + Constant region (conserved within class).

Fab (fragment antigen-binding): Y arms, light chain + heavy chain V+C1 domains. ANTIGEN BINDING. Fc (fragment crystallizable): Y stem, heavy chain constant regions. EFFECTOR FUNCTIONS (complement, Fc receptor binding).

Light chains: Kappa or Lambda — ONE type per antibody molecule, never mixed. Heavy chains: γ, μ, α, δ, ε — DEFINES the Ig class.

5 Ig Classes

Antibody Function

• Neutralization — blocks toxin/virus receptor binding
• Opsonization — coats pathogen, ↑phagocytosis (Fc receptor)
• Complement activation — classical pathway (IgM/IgG-antigen) → MAC → lysis
• Agglutination — clumps particulate antigen (bacteria, RBC)
• Precipitation — clumps soluble antigen
• ADCC — directs nonspecific killer cells via Fc receptor

Primary vs Secondary Response

Primary (1st exposure): lag before Ab detected. IgM first, modest IgG rise. Low titer, short-lived. Secondary/Memory (re-exposure): memory B cells respond fast. IgG dominates from start (prior class switch). Higher, faster, LONGER-persisting titer.

= basis of vaccination/booster dosing.

Antigen

Definition and Properties

An antigen is any substance capable of binding specifically to an antibody or a T-cell receptor. A subset of antigens — immunogens — can additionally trigger an immune response on their own; every immunogen is an antigen, but not every antigen is capable of inducing a response by itself.

Immunogenicity depends on several properties working together. Foreignness matters most — the immune system is built around distinguishing self from non-self, so the degree of an antigen’s difference from the host’s own molecules broadly predicts how strong a response it provokes. Molecular size matters too: large molecules (typically >10 kDa) are generally more immunogenic than small ones, since a larger surface offers more distinct sites for recognition. Chemical complexity — proteins (the most immunogenic class) and polysaccharides are generally strong immunogens; lipids and nucleic acids are weak on their own, becoming immunogenic mainly when carried on a protein or polysaccharide backbone. Physical form also plays a role — particulate and aggregated antigens tend to be more immunogenic than soluble ones, since particulate material is phagocytosed more readily.

Susceptibility to processing is a genuine requirement, not a minor detail: an antigen must be able to be degraded and presented on an MHC molecule for a T-cell-dependent response to occur at all — a property that separates thymus-dependent antigens (needing this processing) from thymus-independent antigens (which can activate B cells directly, without it).

Epitopes and Haptens

An epitope (antigenic determinant) is the specific, small region of an antigen actually recognized by an antibody or T cell receptor — a single large antigen typically carries multiple distinct epitopes, each capable of provoking its own antibody response. B cell epitopes are usually surface-exposed and conformational (dependent on the molecule’s 3D folding); T cell epitopes are linear peptide sequences, since T cells only recognize antigen after it has been processed and unfolded.

A hapten is a small molecule that is antigenic (can bind antibody) but not immunogenic on its own (cannot provoke an antibody response by itself) — it becomes immunogenic only once it is chemically coupled to a larger carrier molecule (typically a protein), which supplies the additional epitopes needed for full T-cell-dependent activation. This is exactly the mechanism behind several clinically important reactions: penicillin allergy and nickel contact dermatitis both arise because the small hapten (the drug or metal) binds host protein to form a hapten-carrier complex that the immune system then treats as foreign.

Classification of Antigens

Antigens are classified along several axes depending on what’s clinically useful to distinguish:

• By origin — exogenous (entering from outside the body, processed via the endocytic pathway and presented on MHC-II) versus endogenous (synthesized within the host’s own cells — including viral proteins made by an infected cell, or tumour proteins — processed via the cytosolic pathway and presented on MHC-I).
• By chemical nature — proteins, polysaccharides, lipids, nucleic acids, each with characteristically different immunogenicity, as above.
• By T-cell dependence — thymus-dependent antigens (most protein antigens; need T cell help, produce memory and class-switched antibody) versus thymus-independent antigens (typically bacterial polysaccharides and LPS; activate B cells directly via extensive cross-linking of B cell receptors, produce mainly IgM, generate weaker memory).
• By relation to the host — autoantigens (self-antigens, normally tolerated), alloantigens (from a genetically different member of the same species — the basis of transplant rejection and blood group incompatibility), heteroantigens (from a different species), and heterophile antigens (antigens shared across unrelated species or genera, cross-reacting despite no close relationship — the basis of tests like the Weil-Felix reaction and the Paul-Bunnell test).
• By source in transplantation/tumour biology — superantigens (microbial products, e.g. staphylococcal toxins, that bypass conventional antigen processing to activate huge numbers of T cells nonspecifically by bridging the TCR Vβ region directly to MHC-II), and tumour antigens (covered in depth under Transplant and Tumour Immunology).

Antibody (Immunoglobulin)

Basic Structure

An antibody (immunoglobulin, Ig) is a glycoprotein produced by plasma cells that binds a specific antigen. The basic unit is a Y-shaped molecule of four polypeptide chains — two identical heavy chains and two identical light chains — held together by disulfide bonds. Each chain has a variable region (at the N-terminus, differing between antibodies, forming the actual antigen-binding site) and a constant region (largely conserved within a class).

Functionally, the molecule splits into two fragments: Fab (fragment antigen-binding) — the two arms of the Y, each containing one light chain plus the variable and first constant domains of a heavy chain, responsible for actual antigen binding; and Fc (fragment crystallizable) — the stem of the Y, made of the constant regions of both heavy chains, responsible for the antibody’s effector functions (complement fixation, binding to Fc receptors on phagocytes and other effector cells).

Light chains come in two types, kappa and lambda, and any single antibody molecule carries only one type or the other, never a mix. Heavy chains come in five types — γ, μ, α, δ, ε — and it is the heavy chain type that actually defines an antibody’s class.

The Five Immunoglobulin Classes

Antibody Function

Antibodies protect the host through several distinct mechanisms working together: neutralization (physically blocking a toxin or virus from binding its target receptor); opsonization (coating a pathogen so phagocytes, which carry Fc receptors, engulf it more efficiently); complement activation (the classical pathway is triggered specifically by antigen-bound IgM or IgG, leading to the membrane attack complex and target lysis); agglutination (clumping particulate antigens, such as whole bacteria or red cells, together); precipitation (clumping soluble antigens out of solution); and ADCC (directing nonspecific cytotoxic cells to an antibody-tagged target via Fc receptor engagement).

The Primary and Secondary Response

On first antigen exposure (primary response), there is a lag before detectable antibody appears, IgM predominates initially with a modest IgG rise following, and overall antibody titre is low and short-lived. On re-exposure to the same antigen (secondary/memory response), memory B cells generated during the primary response respond rapidly, IgG dominates from the start (thanks to prior class switching), and the antibody titre rises higher, faster, and persists longer — the entire practical basis of vaccination and booster dosing.

Short Essays (2)

Short Notes (6)

Multiple Choice Questions (1)