Three lines of defense against pathogens

The human body’s first and second line of defenses act against potentially infectious and harmful agents like microorganisms, chemicals and toxins.

As for nonspecific defenses, there are two ways in which the body puts up a fight to stop pathogens from entering the body: external barriers, and internal defenses. Reference: Campbell; Essential biology with Physiology, Second edition.

The body’s first line of defense against pathogens is to not let them into the body. To stop the pathogens from entering the body, we have our skin, tears, sweat, and saliva. Lysozyme within the tears and saliva, for example, can actually break down some foreign agents to make them harmless.

If some do make it inside the body, another non specific response is with white blood cells, such as macrophages, and neutrophils. In this second line of defense, these cells target anything that they recognize as not belonging to the body. They do not need antibodies to recognize the invaders, but after recognizing a certain pathogen, they will stimulate other lymphocytes to respond. The body also produces defensive proteins, known as interferons (to protect against viral infection), and complement proteins which cause an invading cell to lyse (explode). Reference: Campbell; Essential biology with Physiology, Second edition.

This is a macrophage in a mouse preparing to engulf a foreign cell.

This is a neutrophil

The third line of defense is very specific.

In the third line of defense, antibodies which are soluble proteins, bind to unwanted cells marking them for destruction (as with a virus, for example). In addition, many copies of the antibody for this virus will be made so the virus can be recognized the next time it enters the body. At this level, there are two types of immunity: humoral and cell mediated defenses. In humoral immunity (antibody mediated immunity by B cells and lymphocytes), antigens circulate the body looking for foreign cells, which they mark for destruction by cell eaters such as macrophages or neutrophils. These special binding proteins are produced by B cells.
In cell mediated immunity, T-cells (T-helper and T cytotoxic) work together to kill unwanted cells.

Here we can see red blood cells together with white blood cells and platelets.

Here we can see how the macrophage is engulfing foreign cells, killing them, and pushing them back out (Phagocytosis).

Cell mediated immunity involves the release of certain kids of cytokines. In effect, the cells are releasing substances to kill invading cells, or simply marking them for destruction by macrophages.

Simon R. Downes


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