A Level Biology: Microorganisms and Health
2 – Physical Barrier and Phagocytes Hi! Welcome to my second video on the series about
Microorganisms and Health. What we are looking at today are the methods
by which your body stops you getting ill. The first of which are the physical barriers. These physical barriers include things like
your skin. You probably notice that if you feel your
skin, it got an almost like a layer of oil. Within that liquid that is produced, you have
enzymes and you have chemicals which actually can break down bacteria. We also have tears, which inside these tears
are specific chemicals which can break down microorganisms. Obviously, if you were to ingest or swallow
any microorganism and they are able to get into your stomach which is quite likely, then
the stomach acid, which has a very, very low pH, is able to digest and break down microorganisms
quite effectively.

What would happen if they got through the
physical barriers and then got inside your body, potentially through a cut or through
a wound internally? Then, once it gets to the bloodstream, what
protects you on the inside? We have a specific type of blood cell that
actually deals with foreign infections and these are called white blood cells, hence
the drawing here. In this diagram here, it’s got your red
blood cells and the typical white blood cell here.

Now, the specific name for white blood cells
is called leukocyte. You might have heard the word “leuko”
before in the word leukemia. So leukemia is a cancer of white blood cells. The only time you hear the word leukocyte
that means white blood cell. The first type of white blood cell that the
pathogen might encounter is called phagocyte. The phagocytes work by ingesting and digesting
microorganisms. The diagram here gives you an indication of
how phagocytes work. The blue blob here is a phagocyte and this
is the nucleus of a phagocyte here. The purple section here might be a particular
microorganism, probably bacteria. Since what happens is the bacteria is recognized
as being foreign and then it is engulfed into the phagocyte, what will technically happen
is the phagocyte will form a membrane around the foreign pathogen and it will then release
digestive enzymes that will break down the pathogen and then obviously, eliminate the

In order for the microorganism to be digested,
we need to identify if it is foreign, first of all. Basically, on a surface of a microorganism
within the surface of most cells, you have these things called antigens. These are surface protein markers. They’re a bit like an identification method. So your cells, your red blood cells have a
specific type of antigen and therefore, your white blood cells recognize your red blood
cells as being yours. This is what can lead to problems when people
have blood transfusions from different blood types because different blood types have different
antigens on them, therefore your white blood cells see them as a problem and they therefore
try to digest them and break them down, obviously leading to serious complications. That’s why you can’t mix blood types. But it is also the same reason why your white
blood cells would target certain pathogens because they have antigens on their surface
that are foreign. In summary, you have physical barriers that
stop microorganisms getting inside your body and those are things like your skin, the tears
and the stomach acid.

If they do gain an entry into your body and
into your bloodstream, then the first is met by a type of white blood cell or leukocyte
called a phagocyte. Phagocytes work by digesting and engulfing
microorganisms. If you will join me for the next video, then
we will be looking at the next stage in your immune response. [end of audio – 03:59]
A Level Biology: Microorganisms and Health 2 – Physical Barriers and Phagocytes



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