A macrophage about to engulf the tuberculosis bacteria, “Mycobacterium tuberculosis” (3D illustration)
Organisms must find a means of defense against antigens such a viruses described on the previous tutorial. If this was not the case, bacteria, fungi, and viruses would replicate out of control inside other organisms which would most likely already be extinct.
Therefore organisms employ many types of defense to stop this happening. Means of defense can be categorized into first and second lines of defense, with the first line usually having direct contact with the external environment.
If these first lines of defense fail, then there are further defenses found within the body to ensure that the foreign agent is eliminated.
Second lines of defense deal with antigens that have bypassed the first lines of defense and still remain a threat to the infected organism.
Interferons are a family of proteins that are released by a cell that is under attack by an antigen. These interferons attach themselves to receptors on the plasma membrane of other cells, effectively instructing it of the previous cells’ situation.
This tells these neighboring cells that an antigen is nearby and instructs them to begin coding for antiviral proteins, which upon action, defend the cell by shutting it down. In light of this, any invading antigen will not be able to replicate its DNA (or mRNA) and protein coat inside the cell, effectively preventing the spread of it in the organism. These antiviral proteins provide the organism with protection against a wide range of viruses.
This action brought about by interferon is a defensive measure, while white blood cells in the second line of defense in animals can provide a means of attacking these antigens.
One method of attacking antigens is by a method called phagocytosis, where the contents of the antigen are broken down by molecules called phagocytes.
These phagocytes contain digestive enzymes in their lysosomes (an organelle in phagocytes) such as lysozyme. White blood cells such as a neutrophil or a monocyte are capable of undergoing phagocytosis, which is illustrated below.
The above illustrates one method of ridding an organism of an internal threat caused by an antigen. This is a non-specific response to an antigen. The next tutorial looks at specific immunity and focuses on plant defenses.
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