Sunday, June 14, 2020

Ch 3. Meet your defense forces - immunity to coronaviruses in context of COVID-19

Reading time: 2:07 minutes
Readability: Grade 8

In the previous chapter we saw the consequences of an unchecked viral infection. Before discussing immune function, we need to understand components of the immune system. If we start out by naming the many types of immune cells it can get overwhelming. A function-based approach, lets us appreciate the diversity of immune cells.

There are 2 broad division of the immune response which are further subdivided into other special units (Figure 1)

Figure 1: Function based classification of immune system

1) Innate:

This is an ancient defense system present in all living beings. Plants, fungi, insects, and primitive many-celled organisms have this as the main defense. This early response system is not specific, it can only distinguish what is part of self from enemy. A familiar blood test - WBC count measures the number of white cells in blood, a part of the innate immune system. Its advantage is early response, but it cannot learn and remember past experience.

The cells in this system can be troops or scouts, though each type can do a little of both. Troops have tools to fight and kill infectious organisms. They don’t need extensive training, and develop in large numbers in bone marrow. These properties make them ideal cells to arrive at the scene of an infection early on. A neutrophil is a troop class cell that attacks bacteria that are outside the cell. A “natural killer” cell is troop class, which will kill a virus by killing the cell where virus makes home.

The scouts sample the battlefield to pick up pieces of the dead enemy to identify who has attacked us. This important information is necessary engage special forces of the advanced immune response. Bacteria, virus, fungus, or parasite each need a different advanced immune response. Scouts (antigen presenting cells) are a crucial link between early and advanced response.

2) Adaptive

This immune system engages later in infection. It adapts to the type and strength of infection which is why its called adaptive. A virus causing illness will need different response from bacteria. This system has a cell based response (T cells) to fight infection. The killer cells (CD8 T cells) kill infections (viruses, TB bacterium) that hide inside the cell. 'Helper' cells (CD4 T cells) help, co-ordinate, command the adaptive immune response.

The reason why HIV is so devastating is because the HIV virus eliminates these helpers. By taking out, commanders, HIV causes AIDS and infections that otherwise we can handle.

This system also includes the antibody response. These are proteins produced by B-cells of the adaptive immune system. Antibodies can attack any organisms that are in the blood stream and in tissue but outside of the cells. Antibodies blocks and neutralizes virus before it enters and hides in a cell. Many vaccines work as they designed to induce a strong antibody response.

With coronavirus, we are starting to understand which parts of the immune system engage. The early innate system arrives at site of damage from virus. Killer cells and antibodies play a role. In the later chapters we will see each of these in detail.

Next section: Ch 4. A favorable outcome - immunity to coronaviruses in context of COVID-19
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