Unit 21: Immunity — Short Questions

There are four main kinds of T cells: (1) Helper T cells initiate the immune response by activating other T and B cells; (2) Cytotoxic T cells destroy the body cells that have been infected by microbes or that have become cancerous; (3) Inducer T cells supervise the development/maturation of T cells in the thymus; (4) Suppressor T cells terminate the immune response once the infection has been controlled.

Skin is the outermost layer of the body and forms the first line of defence against microbial invasion. Its epidermal stratum corneum cells form tough, impenetrable keratin that blocks microbe entry, and worn-out cells are continuously replaced from the deep epidermis. In addition, oil and sweat glands lower the skin surface pH to 3–5, which inhibits the growth of many microbes, and sweat contains the enzyme lysozyme, which digests the cell walls of many bacteria.

In the digestive tract, saliva (containing lysozyme) kills many bacteria present in food in the oral cavity, strong acids in the stomach kill bacteria, and protein-digesting enzymes in the intestine destroy them further. In the respiratory tract, ciliated epithelial cells of the nasal cavity trap bacteria before they enter the airway, and cells lining the bronchi and bronchioles secrete sticky mucous that traps microbes; cilia then sweep this mucous upward so it can be swallowed.

Natural killer cells are a type of T-lymphocytes (also called cytotoxic T-cells) that detect and attack body cells infected with viruses rather than attacking microbes directly. They release perforins, proteins that insert into the target cell's membrane and form a pore; water rushes in through this pore, causing the cell to swell and burst. NK cells can also detect and kill cancer cells before they develop into tumours.

Pyrogens are chemicals released by macrophages when they attack invading microbes. They pass through the bloodstream to the brain and reach the hypothalamus, the body's thermostat, where they stimulate it to raise the body's core temperature above the normal 37°C (98.6°F), producing fever (pyrexia), which hinders microbial reproduction and speeds up neutrophil and macrophage activity.

Autoimmune diseases result when the immune system is unable to distinguish the body's own cells from foreign material, so it attacks and destroys the body's own cells and tissues. Examples include myasthenia gravis (antibodies against acetylcholine receptors on skeletal muscle), systemic lupus erythematosus (antibodies against connective tissues and organs), rheumatoid arthritis (antibodies against joint cartilage), and Type I (juvenile) diabetes (antibodies destroy insulin-producing pancreatic cells).

An antigen is a foreign molecule, usually present on the surface of a microbe, that is recognized by T and B cells and that stimulates an immune response against itself. An antibody is a Y-shaped protein produced by plasma cells (derived from B cells) that binds specifically to the antigen that stimulated its production, marking it for destruction.

B cells develop and mature entirely in the bone marrow; on encountering an antigen they proliferate into plasma cells that secrete antibodies, providing antibody-mediated (humoral) immunity. T cells also arise in the bone marrow but migrate to the thymus to mature; they bring about cell-mediated immunity, with cytotoxic T cells directly destroying virus-infected and cancerous cells.

Macrophages develop from monocytes, engulf bacteria by endocytosis and kill them with lysozyme enzymes; some are fixed in organs like the lungs, liver, spleen and brain while most patrol the blood and lymph. Neutrophils are a separate type of white blood cell that kill pathogens by three methods — phagocytosis, releasing soluble antimicrobial chemicals, and releasing networks of DNA fibres that trap and kill bacteria.

Complement system proteins are produced by the liver, circulate inactive in blood, and activate in sequence on encountering a microbe; the final five form a membrane-attack complex that punctures the microbe's plasma membrane, causing it to swell and burst. Interferons are cytokines released by virus-infected cells that cause neighbouring cells to boost their anti-viral defences and that also activate natural killer cells and macrophages.

Innate (inborn) immunity is the non-specific defence present at birth, comprising the first line of defence (skin, digestive and respiratory tract barriers) and second line of defence (germ-killer cells, complement proteins, inflammation, fever). Acquired immunity is the specific, third line of defence developed during growth, which targets particular germs and can remember previous invaders for a faster future response; it is of active and passive types.

In active immunity, the body itself prepares antibodies against the antigens of a microbe, giving long-lasting protection with memory cells. In passive immunity, antibodies made by another organism are given to the individual to combat specific microbes; it acts quickly but is short-lived since no memory cells are formed.

In natural active immunity, the body prepares antibodies naturally when exposed to antigens during an actual infection; it is the most effective form of immunity. In artificial active immunity, weakened antigens are deliberately administered to the body through vaccination, stimulating it to prepare antibodies against the real antigen without causing full-blown disease.

Cell-mediated immunity is carried out by cytotoxic T cells, which directly destroy virus-infected or cancerous body cells by puncturing their membranes. Antibody-mediated (humoral) immunity is carried out by B cells, which do not attack antigens directly but proliferate into plasma cells that secrete antibodies to mark antigens for destruction by complement proteins, macrophages, or killer cells.