Revision NotesAS

immune system — The body’s internal defence system.

This system comprises various cells and molecules that work together to protect the body from pathogens and foreign substances. It distinguishes between self and non-self to mount specific responses, acting like the body's security force with different units and weapons to identify and neutralise threats.

antigen — A substance that is foreign to the body and stimulates an immune response (e.g. any large molecule such as a protein).

Antigens are typically large molecules found on the surface of pathogens or released by them. The immune system recognises these as non-self and produces antibodies or activates T cells against them, much like a unique 'ID badge' on a foreign invader that immune cells recognise as a threat.

self — Refers to substances produced by the body that the immune system does not recognise as foreign, so they do not stimulate an immune response.

The immune system undergoes a maturation process where lymphocytes that react to self-antigens are typically destroyed or inactivated. This prevents the immune system from attacking the body's own cells and tissues. Self-antigens are like the 'uniform' worn by your body's own cells, which the immune system recognises as belonging and therefore doesn't attack.

non-self — Refers to any substance or cell that is recognised by the immune system as being foreign and will stimulate an immune response.

These substances, often antigens, trigger the activation of lymphocytes and the production of antibodies or killer cells. This recognition is crucial for defence against pathogens and foreign tissues. Non-self antigens are like 'enemy uniforms' that the immune system identifies as threats, prompting an attack.

immune response — The complex series of responses of the body to the entry of a foreign antigen; it involves the activity of lymphocytes and phagocytes.

This involves the coordinated action of B-lymphocytes (producing antibodies) and T-lymphocytes (killing infected cells or coordinating the response), often with the help of phagocytes. It leads to the elimination of the antigen and the development of immunological memory, much like a coordinated military operation.

clonal selection — Individual lymphocytes have cell surface receptors specific to one antigen; this specificity is determined as lymphocytes mature and before any antigens enter the body (during an immune response the only lymphocytes to respond are those with receptors specific to antigens on the surface of the invading pathogen).

When an antigen enters the body, it 'selects' the specific B or T lymphocytes that have complementary receptors. This ensures that only the relevant lymphocytes are activated to proliferate, much like finding a specific key that fits only one particular lock (antigen).

clonal expansion — The increase in number of specific clones of lymphocytes by mitosis during an immune response.

Once a specific lymphocyte is selected by an antigen, it undergoes rapid mitotic division to produce a large number of identical cells (a clone). This ensures a sufficient number of effector cells to combat the infection, similar to rapidly making many copies of a key to open all the locks quickly.

plasma cell — Short-lived, activated B-lymphocyte produced during clonal expansion; plasma cells produce and release antibody molecules.

These cells are highly specialised for antibody production, possessing extensive rough endoplasmic reticulum and mitochondria. They secrete large quantities of specific antibodies into the blood and lymph to combat infection, acting like antibody 'factories'.

memory B cell — Long-lived, activated B-lymphocyte that is specific to one antigen; memory cells are activated to differentiate (develop) into plasma cells during secondary immune responses to the specific antigen.

These cells persist in the body for a long time after the primary infection. Upon re-exposure to the same antigen, they rapidly divide and differentiate into plasma cells and more memory cells, leading to a faster and stronger secondary immune response. They are like the 'veterans' of the immune system.

primary immune response — The first immune response to a specific antigen.

This response is relatively slow and produces a lower concentration of antibodies because there are few specific lymphocytes initially. It takes time for clonal selection and expansion to generate enough effector cells, much like the first time your body encounters a new enemy.

secondary immune response — The second and any subsequent immune responses to a specific antigen.

This response is much faster, stronger, and produces a higher concentration of antibodies due to the presence of memory cells. These memory cells rapidly differentiate into plasma cells upon re-exposure to the antigen, acting like a rapid, pre-planned counter-attack.

immunological memory — The ability of the immune system to mount a larger and more rapid response to an antigen that has already been encountered before.

This is the basis of long-term immunity and vaccination. It is mediated by long-lived memory B and T cells that remain in the body after the primary infection, allowing for a swift and effective secondary response, like having a 'most wanted' list for previously encountered criminals.

antibody — A glycoprotein (immunoglobulin) made by specialised lymphocytes in response to the presence of a specific antigen; each type of antibody molecule has a shape that is complementary to its specific antigen.

Antibodies are Y-shaped proteins with two identical antigen-binding sites. They bind to specific antigens, marking pathogens for destruction, neutralising toxins, or preventing viral entry into cells, much like a specific 'key' that fits only one particular 'lock' (antigen).

variable region — Region of an antibody molecule composed of parts of the light and heavy polypeptide chains that form the antigen-binding site; the amino acid sequences of the variable site form a specific shape that is complementary to a particular antigen.

This region is highly diverse among different antibody molecules, allowing each antibody to bind specifically to a unique antigen. The precise 3D shape of the variable region determines its antigen specificity, much like the unique 'teeth' of a specific key.

antigen presentation — The process of preparing antigens and exposing them on the surface of host cells (e.g. macrophages) for recognition by T-lymphocytes.

Macrophages, after engulfing pathogens, process their antigens and display them on their cell surface membranes. This allows T-helper cells to recognise the antigen and initiate a coordinated immune response, like a 'wanted poster' displayed by immune cells.

T-helper cell — Type of T-lymphocyte that secretes cytokines to coordinate activity during immune responses.

Upon activation by antigen-presenting cells, T-helper cells release cytokines that stimulate B cells to divide and differentiate into plasma cells, and also enhance the activity of macrophages and T-killer cells. They are like the 'commanders' of the immune system.

T-killer cell — Type of T-lymphocyte that attaches to cells, releasing toxic substances to kill infected cells and cancer cells.

T-killer cells recognise foreign antigens displayed on the surface of infected body cells or cancer cells. They then bind to these cells and release toxic substances, causing the target cells to die, thus eliminating the pathogens within. They are like the 'assassins' of the immune system.

cytokine — Any signalling molecule released by cells to influence the growth and/or differentiation of the same or another cell.

Cytokines are a diverse group of small proteins that act as chemical messengers between immune cells. They play crucial roles in regulating the intensity and duration of immune responses, cell proliferation, and differentiation, much like 'walkie-talkies' for immune cells.

active immunity — Immunity gained when an antigen enters the body, an immune response occurs and antibodies are produced by plasma cells.

This type of immunity involves the body's own immune system actively producing antibodies and memory cells in response to an antigen. It provides long-term protection because of immunological memory, like your body learning to fight an enemy by engaging in battle.

natural active immunity — Immunity gained by being infected by a pathogen.

This occurs when a person naturally encounters a pathogen, gets infected, and their immune system mounts a primary response, leading to the production of memory cells and long-term immunity, like getting sick and then becoming immune.

vaccine — A preparation containing antigens to stimulate active immunity against one or several diseases.

Vaccines introduce antigens (e.g., weakened pathogens, dead pathogens, or pathogen fragments) into the body without causing disease. This stimulates a primary immune response, leading to the formation of memory cells and long-term immunity, acting like a 'training exercise' for your immune system.

artificial active immunity — Immunity gained by putting antigens into the body, either by injection or by mouth.

This is achieved through vaccination, where antigens are deliberately introduced to stimulate the immune system. It results in the production of memory cells and long-term immunity without experiencing the full disease, like getting a 'drill' or 'simulation' of an enemy attack.

vaccination — Giving a vaccine containing antigens for a disease, either by injection or by mouth; vaccination confers artificial active immunity without the development of symptoms of the disease.

This medical procedure aims to prevent infectious diseases by stimulating the immune system to develop immunological memory. It is a key public health strategy for controlling the spread of diseases, like giving your body a 'mugshot' of a criminal.

passive immunity — The temporary immunity gained without there being an immune response.

This type of immunity involves receiving pre-formed antibodies from another source, rather than the body producing its own. It provides immediate but temporary protection because no memory cells are formed, like receiving 'borrowed' weapons.

artificial passive immunity — The immunity gained by injecting antibodies.

This is used for immediate protection against rapidly acting diseases (e.g., tetanus, diphtheria) or toxins. Antibodies are collected from vaccinated donors and injected into the recipient, providing immediate but short-lived protection, like getting an immediate 'reinforcement' of trained soldiers.

natural passive immunity — The immunity gained by a fetus when maternal antibodies cross the placenta or the immunity gained by an infant from breast milk.

This provides temporary protection to newborns and infants against diseases their mother is immune to. Maternal antibodies (IgG across placenta, IgA in colostrum/breast milk) protect the infant until their own immune system matures, like a mother passing on her 'shield' to her baby.

herd immunity — Vaccinating a large proportion of the population; provides protection for those not immunised as transmission of a pathogen is reduced.

When a high percentage of a population is immune to a disease, it significantly reduces the likelihood of the pathogen spreading, thereby protecting vulnerable individuals who cannot be vaccinated (e.g., infants, immunocompromised). This creates a 'protective bubble' around a community.

ring immunity — Vaccinating all those people in contact with a person infected with a specific disease to prevent transmission in the immediate area.

This strategy is used to contain outbreaks by creating a 'zone of immunity' around an infected individual. It prevents the pathogen from spreading further into the wider population, like creating a 'firebreak' around a small fire.

monoclonal antibody (Mab) — An antibody made by a single clone of hybridoma cells; all the antibody molecules made by the clone have identical variable regions so are specific to one antigen.

Mabs are highly specific antibodies produced in large quantities from a single B cell clone. Their specificity makes them valuable tools in diagnosis (e.g., detecting specific antigens) and treatment (e.g., targeting cancer cells), acting like 'precision-guided missiles'.

hybridoma — A cell formed by the fusion of a plasma cell and a cancer cell; it can both secrete antibodies and divide to form other cells like itself.

Hybridomas combine the antibody-producing ability of plasma cells with the immortality of cancer cells. This allows for the continuous production of large quantities of specific monoclonal antibodies in culture, like a 'super cell' that is an endless factory for specific antibodies.