Antibodies: structure and function

· Antibodies = specific globular proteins produced by plasma cells in response to a specific antigen.
· Basic structure = Y-shaped molecule made from 4 polypeptide chains: 2 heavy chains + 2 light chains.
· Chains are held together by disulfide bonds, helping maintain the antibody’s tertiary/quaternary structure.
· Each antibody has variable regions with specific antigen-binding sites; these are complementary to a particular epitope on an antigen.
· Specificity occurs because the shape of the antigen-binding site matches only one antigen/epitope.
· Hinge region gives flexibility, allowing antigen-binding sites to bind antigens at different angles.
· Constant region allows attachment to phagocytes or other immune system components.
· Antibodies help defend the body by neutralising toxins/pathogens, agglutinating pathogens, and marking pathogens for phagocytosis.
· Agglutination = antibodies bind to antigens on multiple pathogens, clumping them together so they are easier for phagocytes to engulf.

This diagram shows the basic antibody structure, including the antigen-binding sites and chain arrangement. Use it to link antibody shape to specific antigen binding and immune defence. Source

Monoclonal antibodies

· Monoclonal antibodies = identical antibodies produced from a single clone of cells.
· They bind to one specific antigen/epitope, so they are highly specific.
· Useful because they can be made in large quantities and designed to bind to a chosen target molecule.

Hybridoma method for monoclonal antibody production

· Inject a mouse with a specific antigen to stimulate an immune response.
· Mouse produces B-lymphocytes that make antibodies against that antigen.
· Remove B-lymphocytes from the mouse, usually from the spleen.
· Fuse B-lymphocytes with myeloma cells = cancer cells that divide indefinitely.
· Fused cells are called hybridoma cells.
· Hybridomas combine the useful features of both cells:
· B-lymphocyte → produces the required antibody.
· Myeloma cell → divides repeatedly in culture.
· Screen hybridomas to find the cells producing the desired monoclonal antibody.
· Clone the selected hybridoma and culture it to produce large quantities of the antibody.

Monoclonal antibodies in diagnosis

· Monoclonal antibodies can be used to detect the presence of a specific antigen or molecule in a sample.
· A monoclonal antibody may be attached to a marker, such as an enzyme, fluorescent dye, or radioactive label.
· If the target antigen is present, the antibody binds to it and the marker produces a detectable signal.
· Examples include detection of pathogens, hormones, disease markers, or abnormal proteins.
· Key exam idea: monoclonal antibodies are useful in diagnosis because they are highly specific and can identify very small quantities of a target molecule.

Monoclonal antibodies in treatment

· Monoclonal antibodies can be used to treat disease by binding to specific target molecules on diseased cells or pathogens.
· They may block receptors, neutralise molecules, or help immune cells identify and destroy target cells.
· In cancer treatment, monoclonal antibodies may bind to antigens on cancer cells, marking them for destruction.
· Some monoclonal antibodies can be linked to drugs, toxins, or radioactive substances to deliver treatment directly to target cells.
· Advantage: treatment is more targeted, so there may be less damage to healthy cells than with non-specific treatments.

Active, passive, natural and artificial immunity

· Active immunity = the body produces its own antibodies after exposure to an antigen.
· Active immunity involves memory cells, so it is usually long-lasting.
· Passive immunity = antibodies are received from another source; the body does not make them itself.
· Passive immunity gives immediate protection, but it is usually short-term because no memory cells are formed.
· Natural active immunity = immunity after infection by a pathogen.
· Artificial active immunity = immunity after vaccination.
· Natural passive immunity = antibodies passed from mother to fetus across the placenta or to baby in breast milk.
· Artificial passive immunity = injection of antibodies, such as antitoxins or immune serum.

Vaccines and long-term immunity

· Vaccines contain antigens that stimulate an immune response without causing serious disease.
· Vaccine antigens may come from weakened pathogens, killed pathogens, pathogen fragments, toxins made harmless, or genetic material coding for an antigen.
· Vaccination causes clonal selection of specific lymphocytes.
· B-lymphocytes differentiate into plasma cells and memory cells.
· Plasma cells produce specific antibodies during the primary immune response.
· Memory cells remain in the body and allow a faster, stronger secondary immune response if the real pathogen is encountered later.
· Secondary response produces antibodies more quickly and in higher concentration, often destroying the pathogen before symptoms develop.
· Key exam phrase: vaccines provide long-term immunity because they stimulate the production of memory cells.

This diagram helps connect vaccination to the primary immune response, including activation of lymphocytes. It supports the idea that first exposure to antigen leads to antibody production and memory cell formation. Source

Vaccination programmes and control of infectious disease

· Vaccination programmes reduce the number of susceptible individuals in a population.
· If enough people are vaccinated, transmission of the pathogen is reduced because infected people are less likely to contact susceptible hosts.
· This can produce herd immunity, where even unvaccinated or vulnerable individuals gain indirect protection.
· Vaccination programmes are most effective when they achieve high coverage across the population.
· They help control infectious disease by reducing infection rates, disease outbreaks, hospitalisations, and deaths.
· Booster vaccinations may be needed if immunity decreases over time or if the pathogen changes by mutation.
· Vaccination can contribute to disease elimination or eradication if there is widespread, sustained immunity and no major non-human reservoir.

This diagram shows why high vaccination coverage protects both vaccinated and some unvaccinated people. It is useful for explaining how vaccination programmes reduce pathogen transmission. Source

Common exam comparisons

· Active immunity: body produces antibodies; memory cells formed; slower initial protection; long-lasting.
· Passive immunity: antibodies supplied from outside; no memory cells formed; immediate protection; short-term.
· Natural immunity: acquired through normal biological processes, such as infection or maternal antibody transfer.
· Artificial immunity: acquired through medical intervention, such as vaccination or antibody injection.
· Monoclonal antibodies: identical antibodies; bind one specific antigen/epitope; useful in diagnosis and targeted treatment.
· Polyclonal antibodies: mixture of antibodies from different B-cell clones; bind different epitopes on the same antigen.