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Principles of Immunology

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Immunity against invading pathogens is a crucial part of maintaining health. The body has adaptations which prevent invasion by pathogens, as well as processes in place to deal with those that manage to penetrate the body's primary defenses. The skin and mucous membranes (e.g. mouth) are examples of such defenses. Sweat contains lysozyme which is an enzyme that breaks down bacterial walls.

If pathogens do invade the body, the subsequent immune response is split between:

1. Non-specific
2. Specific

The non-specific immune response is inflammation and phagocytosis


Phagocytosis

White cells (the most common ones are neutrophils) engulf any foreign particles such as dust or bacteria, then digest them and dispose off of the remains. It's badass, trust me. I've got proof:


The enzymes used to break invaders down are lysosomes which fuse with the vesicle which contains the bacteria. All this action happens within the white cell. At the end, the undigested leftovers are disposed off of by exocytosis (kind of like a burp).

The specific immune response is split into humoral immunity and cell-mediated immunity. Humoral is to do with the blood and antibodies. Distinguishing between an antigen and an antibody is very important.

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Antigen = protein or carbohydrate foreign (not normally present) to a host's organism
Antibody = protein made as a response to detecting an antigen which binds to the antigen and prevents the pathogen from harming the host.



Antibodies are made by B cells or T cells which come from stem cells from bone marrow. B cells release antibodies, while T cells secrete antibodies which stay on the surface of the cell.

B cell --> O - -   -   - - -
T cell --> O-

where "-" is an antibody. Apologies for the horrendous visual representation.

So when a bacterium invades, B cells would release antibodies with a shape complementary to that of the bacterium's antigen. This antibody would then bind to the antigen. T cells on the other hand would secrete the antibodies on their surface, then personally greet the bacterium and bind to it via the antibody. You could say the B cell is shooting the bacterium, while the T cell is strangling it. But for goodness' sake, don't write that in the exam.

When a pathogen invades the body and a B cell releases the appropriate antibody to manage the infection, it's not just the one B cell. They come in their thousands, they are clones of a B cell with a specific antibody, and they are called plasma cells. Plasma cells release a high amount of antibodies, but they are short-lived. Other cells called memory cells may survive for much longer, up to several years. Memory cells are involved in the secondary immune response which happens if a high enough amount of antigens are present. The memory cells replicate into a large number of plasma cells which then release enough antibodies.


Common flu

OK, so if we have all these fancy cells doing our work for us, how come the cold virus gets us again and again and again? Surely our memory cells could identify the cold virus, replicate and defeat it?

Memory cells are specific to certain antigens. The flu virus has many different variations of antigens which change constantly, so by the time we've acquired some resistance to this year's antigen, a new one will have emerged.


Vaccinations

Vaccinations prevent symptoms of an illness (such as flu) from developing, by creating a primary immune response to an unharmful substance that the body identifies as a pathogen. This could be an antigen, or the pathogen itself - dead or otherwise modified to prevent disease. Some vaccines are really successful and have prevented many diseases so far, yet the flu vaccine remains a challenge due to the above points. The virus changes its antigens, and there is great variation to start off with.


Monoclonal antibodies

These are antibodies which can be cloned from a single cell to make a high amount of them. They can bind to pretty much any substance, and are used in pregnancy tests as well as cancer treatment.The process involves taking a cell which produces antibodies such as a lymphocyte, and crossing it with a tumour cell. Tumour cells divide uncontrollably, so the end hybrid cell will produce many antibodies via its many clones.


How Science Works

You need to be able to evaluate data regarding the use of vaccines and monoclonal antibodies, as well as discuss the ethical issues arising from the testing and use of vaccines.
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