The Immune System

Innate immunity is the body's non-specific, immediate, first line of defence against pathogens. It is present from birth and does not require prior exposure to a pathogen. Components include physical barriers (skin, mucous membranes, cilia, stomach acid), phagocytic cells (macrophages, neutrophils), natural killer cells, the inflammatory response, fever, and antimicrobial proteins (e.g., interferons). It acts rapidly but does not confer long-term memory.

Adaptive (or acquired) immunity is the body's specific, slower, but highly effective defence mechanism that develops after exposure to specific pathogens. It is characterised by its specificity (targets particular antigens), memory (remembers previous encounters for a faster, stronger secondary response), and ability to distinguish self from non-self. It involves lymphocytes (B-cells and T-cells) and is divided into humoral immunity (antibody-mediated, primarily by B-cells) and cell-mediated immunity (T-cell-mediated).

An antigen is any substance (typically a protein or polysaccharide) that, when introduced into the body, is recognised as 'non-self' and stimulates an immune response, leading to the production of antibodies or sensitised T-cells. Antigens are often found on the surface of pathogens, foreign cells, or toxins.

Antibodies are Y-shaped proteins produced by plasma cells (differentiated B-lymphocytes) in response to the presence of specific antigens. They bind specifically to the antigen that triggered their production, marking the pathogen for destruction by other immune cells, neutralising toxins, or preventing pathogen attachment to host cells. Each antibody has a unique variable region that binds to a specific epitope on an antigen.

A vaccine is a biological preparation that provides active acquired immunity to a particular infectious disease. It typically contains a weakened (attenuated) or killed form of the pathogen, its toxins, or one of its surface proteins (antigens). When administered, the vaccine stimulates the immune system to produce a primary immune response, including memory cells, without causing the actual disease. This prepares the body for a rapid and effective secondary immune response upon subsequent exposure to the actual pathogen.

Active immunity occurs when an individual's own immune system produces antibodies and memory cells in response to an antigen. This can be natural (e.g., after infection) or artificial (e.g., after vaccination). It is long-lasting. Passive immunity occurs when an individual receives antibodies produced by another organism. This can be natural (e.g., antibodies passed from mother to foetus across the placenta or via breast milk) or artificial (e.g., injection of antiserum containing pre-formed antibodies). Passive immunity provides immediate protection but is short-lived as the body does not produce its own memory cells.

The ABO blood group system classifies human blood based on the presence or absence of two specific antigens (A and B) on the surface of red blood cells, and the presence of corresponding antibodies (anti-A and anti-B) in the plasma. There are four main blood groups: Group A (A antigens, anti-B antibodies), Group B (B antigens, anti-A antibodies), Group AB (A and B antigens, no antibodies), and Group O (no antigens, anti-A and anti-B antibodies).

The Rhesus (Rh) system is another important blood group system, primarily determined by the presence or absence of the Rh factor (D antigen) on the surface of red blood cells. Individuals with the D antigen are Rh positive (Rh+); those without it are Rh negative (Rh-). Unlike the ABO system, anti-Rh antibodies are not naturally present in the plasma of Rh- individuals; they are only produced if an Rh- person is exposed to Rh+ blood (e.g., through transfusion or during pregnancy). This is crucial in preventing Haemolytic Disease of the Newborn (HDN) where an Rh- mother carrying an Rh+ foetus may produce antibodies that attack the foetal red blood cells.

For a safe blood transfusion, the donor's red blood cell antigens must not react with the recipient's plasma antibodies. Incompatible transfusions lead to agglutination (clumping) and haemolysis (bursting) of red blood cells, which can be fatal. Group O is considered the 'universal donor' as its red blood cells lack A and B antigens, so they won't be attacked by anti-A or anti-B antibodies. Group AB is the 'universal recipient' as its plasma lacks anti-A and anti-B antibodies, so it won't attack A or B antigens. Rh compatibility is also vital; Rh- recipients should ideally receive Rh- blood, especially for women of childbearing age.