A disease is impairs a cell or organism’s normal functions. They can be infectious (can be spread) and/or contagious (spread by touch). A symptom is evidence of this disease.

There are three lines of defence against diseases. The first line is non-specific and prevents entry of the invading pathogen (disease causing cell). It includes physical barriers like skin and mucous membranes, and acidic secretions such as those from the stomach, eyes (tears), urethra (urine), skin (sweat), vagina, and several others.

The second line is also non-specific and attacks all invaders. It includes NK cells, complements, fever, inflammation, pus, etc, and macrophages, which are the link between the second and third line of defence. Disease can enter via damaged skin, through inhalation, through sexual transmission, and touch (especially fungus).

The immune response is considered a homeostatic mechanism.

The third line is specific – it attacks specific invaders with specific cells of two different types, B and T, and has a memory of past invaders. There are two types of response to a pathogen – antibody mediated and cell mediated.

They are both initiated by a macrophage. A macrophage is involved in both specific and non-specific immunity, as are lymphocytes (lymphocytes make up 20-30% of white blood cells, and weigh ~1kg at a time). An immature macrophage is called a monocyte and travels in the blood. It leaves the blood to become a macrophage. It uses phagocytosis to engulf antigens.

An antigen is anything that is capable of causing an immune response. They are large molecules, and can be proteins, lipids, or carbohydrates, a foreign cell or part of a cell. Each individual has ‘self-antigens’, which are covered with the individuals Major Histocompatability Complex (MHC) which identifies them as the individual’s cells. Any cell with the wrong MHC is called ‘non-self’ and attacked.

Before birth, the immune system is programmed to differentiate between self and non self. In the second line of defence, a macrophage is the first to attack a non-self antigen. It engulfs the antigen, and covers itself with the MHC of the antigen. Macrophages can also be called Antigen Presentation Cells (APC) - after the antigen is engulfed, it is presented by the macrophage to a T-helper cell. The T-helper cell then sensitises a B cell. Macrophages also ‘clean up’ left over fragments of cells and complexes.

Antibodies, a protein, are produced as a response to a non-self antigen. Antibodies are immunoglobins (Ig), and belong under different classes – IgA, IgD, IgE, IgG and IgM.

Antibody mediated (or humoral) immunity acts against bacteria, toxins, and viruses before they infect a cell, and against red blood cells of a different blood type. It is the first type of immunity to attack a disease. The Antigen Presentation Cells (APC, macrophages) reach the lymphoid tissue, and through a T helper cell, sensitises the B lymphocyte that responds to the invading antigen. That B cell divides into mostly plasma cells and some memory cells.

Plasma cells produce antibodies, which travel through the blood and lymph to the site of infection. The antibodies form an antigen-antibody complex (lock and key), to inactivate or destroy the antigen. Memory cells remain, from the primary response, in the lymphoid tissue in case of second invasion, to more rapidly produce a secondary response.

Without memory cells, it can take a few days for the number of B cells to be effective enough to respond to the disease. With memory cells, antibodies can accumulate so quickly that the disease is barely noticeable.

Antibodies can bind to the antigen’s cell surface to highlight it, agglutinate (clump) cells together, make soluble antigens insoluble, prevent viruses from entering cells, and inhibit antigen reactions.

Cell mediated responses target cells that have been infected by bacteria, viruses, and transplanted tissues, organs and cancer cells. It also resists fungi and parasites. The response begins when the macrophage (APC) activates a T helper cell. The T helper cell then sensitises the B lymphocyte. The matching T cell is then sensitised, and multiplies.

It will form either Killer (or Cytotoxic) T cells, Helper T cells, Suppressor T cells or memory cells. Killer T cells destroys by attaching to the antigen’s surface and releasing a cytotoxic chemical. This creates perforations in the cell wall and causes cell lysis. The Killer T cells can be reused. Helper T cells are created, to sensitise lymphocytes at and attract macrophages to the site of infection, and increase the amount and intensity of phagocytosis.

Suppressor T cells act if the T cell response is excessive or if the threat of the disease is gone. They suppress B and T cell activity to reduce the response. The memory cells, like in an antibody mediated response, stay in the lymphoid tissue  for a more rapid secondary response.

The lymphoid tissue produces and stores most cells of the immune system and monitors the body’s extracellular fluid for pathogens. Lymphoid tissue is found in the lymph nodes, and also in the spleen, tonsils and thymus. B and T lymphocytes are both made in the bone marrow. The B cells mature in the bone marrow then go to the lymphoid tissue, whereas T cells mature in the thymus then go to the lymphoid tissue. This is where they are released from when a disease occurs.

There are four types of immunity – active and passive, and natural and artificial.

Immunisation is natural or artificial catching of a disease, where the body develops memory cells to it. The memory cells create a more rapid secondary response if the body is exposed to the disease again.

A vaccine is the artificial preparation of an antigen. They can contain either live attenuated (weakened) or dead pathogens, toxoids, toxins emitted by the bacteria that have been inactivated, or subunits, parts of the bacteria cell wall. These are used to reduce the virulence of the disease – the ability of the pathogen to cause symptoms of the disease. Attenuated pathogens, although more risky, are more effective than other methods of vaccination. In people with weak immune systems, the vaccine can cause the disease instead of immunising against it, if the virulence is too high.

Altered DNA can also reduce virulence. Recombinant DNA is where DNA is taken from the pathogen and put into harmless bacteria, so it displays the MHC of the pathogen. Memory cells develop to respond to the pathogen. Vaccines can be injected, taken orally, breathed as mist, or put on as a skin patch.

Through vaccines, diseases like smallpox and polio are virtually non-existent. Herd immunity is created- when enough people are immunised, those without immunisation are protected as well as they have nowhere to get the disease from.