Immunity

The body’s ability to fight disease-causing organisms is called immunity. This is not a single mechanism but a coordinated system of cells, tissues, and chemical signals that recognise and neutralise foreign invaders. The immune system is what keeps you alive despite constant exposure to bacteria, viruses, fungi, and parasites.

Immunity is broadly classified into two types: innate immunity and acquired immunity.

Innate Immunity

Innate immunity is the non-specific defence that you are born with. It is present from birth and does not require prior exposure to a pathogen. It acts as the first line of defence, responding immediately to any foreign threat. Innate immunity has four main types of barriers:

• Physical barriers: The skin is the most obvious physical barrier — it prevents microbes from entering the body. Mucous membranes lining the respiratory, digestive, and urogenital tracts trap microbes, and the mucus is then expelled (e.g., by coughing or sneezing).
• Physiological barriers: These are chemical conditions that inhibit microbial growth. Stomach acid (HCl) kills most ingested bacteria. Lysozyme, an enzyme present in tears, saliva, and nasal secretions, breaks down bacterial cell walls.
• Cellular barriers: Certain white blood cells (leukocytes) directly attack pathogens. Neutrophils and monocytes (which become macrophages) engulf and digest microbes by phagocytosis. Natural killer (NK) cells destroy virus-infected cells and tumour cells.
• Cytokine barriers: Infected cells secrete proteins called interferons. Interferons signal neighbouring healthy cells to produce antiviral proteins, preventing the virus from replicating in those cells. This is a chemical, not cellular, defence.

Acquired Immunity

Acquired immunity, also called adaptive or specific immunity, is not present at birth. It develops only after exposure to a pathogen (or a vaccine). It is highly specific — it recognises and targets particular antigens (foreign molecules) — and it has memory: a second encounter with the same pathogen produces a faster and stronger response.

Acquired immunity is carried out by two types of lymphocytes (a kind of white blood cell):

• B-lymphocytes (B-cells): These mature in the bone marrow. When activated by an antigen, B-cells differentiate into plasma cells that secrete antibodies (immunoglobulins) into the blood and lymph. This is called humoral immune response (antibody-mediated).
• T-lymphocytes (T-cells): These mature in the thymus gland. T-cells do not produce antibodies. Instead, they directly attack infected cells or help other immune cells. This is called cell-mediated immune response.

Active and Passive Immunity

Acquired immunity can be further divided based on how it is obtained:

• Active immunity: The body produces its own antibodies and memory cells after exposure to an antigen. This can happen naturally (when you get an infection and recover) or artificially (through vaccination). Active immunity is long-lasting, sometimes lifelong.
• Passive immunity: Ready-made antibodies are transferred from an external source. The recipient’s immune system does not produce them. This can happen naturally (antibodies from mother to foetus via the placenta, or to infant via colostrum) or artificially (injection of antibodies, e.g., anti-tetanus serum). Passive immunity is immediate but short-lived — it lasts only as long as the antibodies remain in the body.

Vaccination

Vaccination is the deliberate introduction of a weakened or killed pathogen (or its antigenic components) into the body. This triggers an active immune response — the body produces memory cells — without causing the actual disease. If the real pathogen later enters the body, the immune system mounts a rapid, strong response. This principle is the basis of immunisation programmes that have eradicated smallpox and nearly eliminated polio.

Structure of an Antibody

An antibody is a Y-shaped protein molecule. Each antibody molecule has four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulphide bonds. The tips of the Y arms form the antigen-binding site — this region is highly variable, allowing millions of different antibodies to recognise different antigens. The stem of the Y is the constant region, which determines the antibody's class (IgG, IgA, IgM, IgD, IgE) and its function in the body.

Immune System in Health

The immune system is not always beneficial. Sometimes it malfunctions:

• Allergies: The immune system overreacts to harmless substances (allergens like pollen, dust, or certain foods). This triggers the release of histamine from mast cells, causing symptoms like sneezing, itching, and swelling. …