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Immune Response

Natural Defence

We are constantly being bombarded with micro-organisms that could be harmful to us. The body has a number of adaptations that stop these ever gaining access to living cells. These natural defence mechanisms include:

SkinThis is generally impenetrable as the outremost layer, the epidermis, is toughened by collagen and keratin - an indigestible protein. On the skin surface are groups of bacteria called commensals, they compete (more successfully) with pathogenic bacteria.
TearsThe eyes are protected from pathogens by the enzyme lysozyme which breaks down cell walls.
SalivaSaliva also contains lysozyme and pathogens swallowed are effectively destroyed in the stomach.
MucasProduced by cells in the respiratory tract. Mucas traps microorganisms that may be breathed in.
CilliaSmall hair-like projections on the epithelial cells in the various parts of the body. The cillia move the mucas.

If pathogens manage to get past these primary defences the body's cells will start their response. The first type of response will be non-specific phagocytosis. This process is carried out by a number of different white blood cells, the most common types being the neutrophils and macrophages.

Once it has 'detected' foreign material, the neutrophil will attach to the microorganism wall and then engulf it and enclose the microorganism in the vacuole. Enzymes from the Golgi body are secreted, that digest the microorganism.

Diagram depicting phagocytosis

Macrophages perform the same process but do so outside of the blood vessels: so they might remove pathogens from the tissue fluid.

Cells have molecules in their wall that identifies them - this is important so the body can identify foreign cells; so these are a bit like a passport. When these markers cause an immune reponse they are called antigens. They can be proteins, polysaccharide or glycoprotein (a combination of protein and carbohydrate).

Antigens are on the surfaces of microorganisms and viruses, allowing them to be detected. Tissue or even entire organs that are transplanted have normal cell markers, but when they aren't your own, they cause the immune system to attackthe tissue - this is why people receiving transplants need to take immunosuppressants. Individual chemicals can also be antigens, toxins for example, but some people may treat harmless chemicals as antigens and so have an allergic response to them.

Cell Mediated Immunity: Lymphocytes

We now move in to look at specific immunity and the role of the T lymphocytes. They are made from stem cells in the bone marrow (soft tissue in bones). They then pass to the thymus gland (sits above the heart), where the T-lymphocytes acquire their receptors specific to an antigen.

The T lymphocytes travel in the lymphatic system until they recognise their specific antigen - at this they divide rapidly and produce many clones of themselves. A type of T cell called killer T lymphocytes bind to the pathogen and produce cytotoxin that destroys cells.

Other types of lymphocyte are produced, including helper T-cells that stimulate macrophages and B cells; memory cells that cause immunity; and suppressor T cells that slow down the immune response about 1 week after the response.

Antibodies and B Lymphocytes

B lymphocytes are also made in the bone marrow and mature in lymph nodes where they acquire membrane receptors. The rapid reproduction of a specific B cell is caused by a macrophage that has an antigen in its membrane coming into contact with the B cell. The large number of cloned B cells secrete antibodies that kill the microorganism.

Antibodies have the following structure.

the structure of an antibody

Antibodies (or immunoglobulins) are made of chains of proteins joined by disulphide bridges: a very strong bond. The key part is the antigen-binding sites which binds to antigens, a bit like an enzyme does. Because each antibody has two binding sites, it can bind to two different pathogens, this cause large, lattice-like, structures called antibody-antigen complexes to form.

diagram of an antibody antigen complex

Antibodies cause different outcomes depending on their type, outlined below are some of these.

Agglutination of antigens.Lots of microorganisms stick together, this is called agglutination. it makes it easier for macrophages to detect and engulf them.
Stimulation of phagocytosis.The antibody causes phagocytosis (see above) to occur.
Precipitation of ToxinsAntibodies cause the soluble toxin to precipitate (become solid or not dissolved) and so it is harmless.