Biology sac 4
The third line of defense in the body, after natural immunity and non-specific immunity, is specific
immunity. This only occurs when the previous methods of defense fail.
Specific immunity is a response by the immune system to a particular antigen. The Heroes of Specific
immunity are B and T cells. These are the two main groups of lymphocytes in the body, and as such
are also known as B and T lymphocytes. Once immunity is acquired, it is usually long lasting or even
for life. This is due to memory cells and is the main focus behind vaccination. Since the immune
system has to target foreign cells in the body, a system is needed to discover which cells are self and
non-self.
Self and non-self:
Every cell in the body has specific proteins on the surface of their cell membrane which acts as a
marker. The proteins are determined by genes, so each person has different markers, and these
unique markers make up the major histocompatibility complex, or MHC. This way, every single cell
has MHC Markers on their surface; which allows immune cells to distinguish between self and nonself. Self cells will have our MHC marker; whist non-self-antigens will not have a different marker,
which will register it as foreign, and an immune response begins. B and T cells are able to distinguish
between self and non self cells due to receptors on their membrane. They have receptors that
recognise self and non-self markers. As you will learn soon, not every B and T cell has the same
receptors to detect non-self MHC markers, because MHC markers change from antigen to antigen.
What is an Antigen?
An antigen is a toxin or other foreign substance that induces an immune response. They have unique
shapes, react with specific B and T cells, Stimulate the production of specific antibodies, and are
often made of protein, but some are polysaccharides.
The main focus of specific immunity is B and T cells.
B and T cells:
B and T cells are both lymphocytes, which are special types of white blood cells. B and T cells are
produced in the bone marrow. However, T cells leave the bone marrow before they are fully
developed and then travel to the thymus gland. Here is where they differentiate with B lymphocytes
to form T lymphocytes, which stands for thymus-dependent. B cells remain in the bone marrow until
fully developed. It is the stem cells in bone marrow which create all types of blood cells.
This differentiation happens for a reason, and as you will soon discover, B and T cells both have very
different functions.
B cells:
B cells have immunoglobulins on their surface. These are proteins designed to detect antigens.
Another name for immunoglobulins is antibodies. The Antibodies on the surface of B cells have a
specific structure, and as a result are only able to detect one kind of antigen. However, there are
millions of different antigens our body must respond to. When B cells are maturing in the bone
marrow, only a few of each kind of B cells is produced, because genetic material undergoes change.
This is why there is diversity within B cells. This way, there is now millions of different B cell, with
different antibodies, with the capability to detect different antigens.
However, this poses a serious problem. If only a small amount of B cells are produced, how are they
supposed to fight off an invading pathogen? Specific immunities answer to this is clonal selection.
clonal selection ( B cells)
The clonal selection hypothesis is a widely accepted model for the way the immune system responds
to infection. It also shows how certain B and T lymphocytes respond to specific antigens that are
invading the body.
Since the antibodies on the surface of the B cell can only combine with certain antigens, it is likely
that an antigen passes by many B cells before it finds one it can combine with. In this sense, the
antigen ‘selects’ the B cell that will lead to its death. The selected B cell then reproduces rapidly to
give rise to a number of identical B cells. Each of the reproduced cells also rapidly reproduces to
form a larger clone of cells. This process is known as clonal expansion. Each of these cells will have
the same specific antibody. Most of these cells with differentiate into plasma cells. These plasma
cells will produce the same kind of antibody, which is released into the blood stream or lymph
system. The distinction between B cells and plasma cells usually occurs in the lymph nodes, in the
germinal centre. Specific immunity is not without delay, as clonal expansion must happen before
there are a sufficient number of plasma cells to fight off the infection. This is why a person may
become quite ill, because the infecting pathogen produces large numbers quickly, before the
antibodies can react with it.
Not every B cell turns into a plasma cell, some also turn into B memory cells. This differentiation also
occurs in the germinal centre. B-memory cells have the same antibody-antigen specificity as the
parent B cell, but unlike the Plasma cells which only live a few days, B-memory cells can live years or
even a life time. This is how your body acquires immunity, as memory cells can react far quicker and
more vigorously against its specific antibody.
If more cells are produced than needed during clonal selection, they are normally killed off through
apoptosis.
T cells:
T cells mature in the thymus. There are many different kinds of T cells which recognise many
different antigens. Like B cells, only a few of each kind of T cells are produced, with different
immunglobulins on their surface, so they can detect specific MHC markers. After encountering a
specific antigen, T cells reproduce in the same way as B cells. T memory cells are formed. However, T
cell does not make Antibodies, and thus there are no T plasma cells. There are different kinds of T
cells, and each reacts with a different part of the body. When T cells and phagocytes are involved in
immunity it is known as specific immunity.
Phagocytes are a type of cell within the body capable of engulfing and absorbing bacteria and other
small cells or particles.
There are three main types of T cells. T cells themselves, which have receptors on their surface that
enable them to recognise non-self markers.
The second is Helper T (Th) cells, these are regulatory cells which stimulate the production of other T
and B cells by secreting a protein. B cells will not reproduce and form plasma cells without the
assistance of Th cells. Th cells also release chemicals which attract other white blood cells to areas
containing antigens. These cells do not kill host cells or pathogens; they activate and direct other
immune cells. Helper T cells are important in the activation of cytotoxic T cells.
Cytotoxic T (Tc) cells kill body cells that have been infected by a virus. This is because the infected
body cell has foreign antigen MHC markers on its cell membrane/surface as well as self markers. Tc
cells kill the infected cell through secreting proteins which punch holes in the membranes of cells,
allowing their contents to ooze out.
Memory Th cells and Memory Tc cells are made during the initial response to the antigen.
How do B and T cells get around the body?
As was mentioned, B cells develop in the bone marrow, and T cells leave the bone marrow before
mature to develop in the thymus. The Bone marrow and Thymus are lymphoid tissue.
To travel around the body, B and T cells will enter the bloodstream, but they can leave it and move
around the body. Other lymphatic tissues and organs include the spleen, lymph nodes and tonsils.
These are connected through a series of lymphatic vessels. Many different types of white blood cells
are packed into these lymphatic tissues. Lymphatic tissues contain lymph. Lymph is another way in
which B and T cells can travel around the body, in particular, memory B and T cells. Lymph drains
from nearby tissues. Antigens that enter the body are often carried in the lymph, where, if they are
not destroyed along the way, are taking to a lymphatic organ full of white blood cells, and the
immune response begins.
Difference between Non-specific and specific defense
Non-specific defense system are mechanisms such as the skin, the chemicals in our perspiration,
saliva, tears, the hairs in our nostrils. These all form physical barriers to protect us against disease.
Another example of non-specific defense would be fever, in which we raise our body temperature to
inhibit the growth of pathogens. As can be seen, this varies immensely from the specific defense
system, since it does not actually target a pathogen, hence the name.