How the Immune System Reacts to Antigen at a First and a Later Exposure

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The graph shows the response of the human immune system to the exposure of an antigen in two phases. The first phase is that of a primary response to the initial attack. The second response is very rapid, subsequent to attacks from the same antigen. The specificity of this system relies heavily on two groups of cells: B lymphocytes and T lymphocytes.

In period I, the antigen is injected at zero days. This is where the antigen will attach itself to a macrophage. By forty days however, the B cells have made antibodies to protect the body from this foreign substance. Antibodies are complex globular proteins also known as immunoglobulins. They make precise three-dimensional combinations with particular molecules that the body recognizes as foreign. At any given time in the body, there are approximately 2 trillion B cells present. The daughter cells resulting from B cell activation differentiate into two types.

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One of these types of cells that are being made in period I, is the plasma cell, a specialized antibody factory. It can make up to 30,000 antibodies a second.

The second type of cell being made in period I is a memory cell. They also produce antibodies, however they live for long periods of time (up to a lifetime), compared to a plasma cell that lives only a few days. Memory cells are made so they can recognize this same antigen if it were to ever reenter the body again.

Interactions between T cells are also known as the cell-mediated response. T cells are not just active against viruses and bacteria, but they also interact with eukaryotic cells. There are three types of T cells: helper T cells, suppressor T cells, and cytotoxic T cells. The T cell receptor (helper T cells) recognizes foreign antigens displayed on the surface of the bodys own cells. When the cytotoxic T cell encounters this, it differentiates into active cells that attack and lyse the infected cells. They also release powerful chemicals known as killer cells.

In period II, the antigen is again injected, however the memory cells recognize it and they immediately begin to produce antibodies, preventing further multiplication of the pathogen. This rapid response of the memory cells is the source of immunity to many infectious diseases such as smallpox, measles, mumps, and polio, following an infection.

By the end of the period, the body has produced enough cells to fight off this antigen and prevent it from doing any harm.

An infection by the AIDS virus (HIV) affects the functions of both the B and T lymphocytes. AIDS is a retrovirus. The problem with this virus is that is a prefect three-dimensional match to the T4 molecules that characterize helper T cells. When the AIDS virus meets a helper T cell, the T4 molecule functions as a receptor and lets it enter the cell. This virus is also able to infect macrophages. As the virus replicates, it bursts the T cells, leaving the body with virtually no T cells. However, before all of the T cells are depleted, B cells respond to the foreign antigens producing antibodies. This will leave the bodys immune system incapable of responding to other infections.

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