The Human Immune System
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The Human Immune System

The human immune system can roughly be divided into two classes of immune responses; humoral and cellular immunity. This article explains both and discusses the primary and secondary immune response.

The human immune system is made up out of a couple of different components and uses a variety of mechanisms to protect the body from invaders. Yet, most immune responses can be grouped into one of two classes:

  • Humoral immunity, and
  • Cellular immunity.

Even though it is easier to consider these to be separate systems they are, in reality, interacting and have a noticeable influence on each other.

Humoral Immunity

One of the two classes of responses of the human immune system is humoral immunity centers on the production of antibodies by special lymphocytes, called B cells, which mature in the bone marrow. Antibodies are proteins that circulate in the blood and other bodily fluids. They bind to specific antigens and mark them for destruction by phagocytic cells, another type of immune cells, responsible for the destruction of invading particles.

Furthermore, antibodies also activate a set of cells, called the complement, that help destroy invading organisms and attract macrophages.

Cellular Immunity

Cellular immunity, the second class of response in the human immune system, is focused around the activity of T cells, which are specialized lymphocytes that mature in the thymus (a specialized organ in the immune system, composed out of two identical lobes and located in front of the heart) and respond only to the antigens that are found on the surface of the body’s own cells.

After a pathogen (let’ say a virus) has infected a cell, viral antigens usually appear on the surface of the cell. Proteins, called T cell receptors, on the surface of the T cells bind to these antigens and mark the infected cell for destruction. These T cell receptors must simultaneously bind a foreign antigen and a antigen from the body itself, known as the major histocompatibility complex (or MHC) antigen.

Not all T cells, however, attack cells with foreign antigens. Some help in regulating the immune response by providing a line of communication between the different components of the immune system.

Primary and Secondary Immune Response

Each lymphocyte is specific for one antigen, and when an pathogen enters the body, only those lymphocytes that can recognize its antigen can bind to it. By doing this, they are stimulated to divide, producing large numbers of lymphocytes for that specific antigen. This is called the primary immune response.

After dealing with the pathogen, most of these lymphocytes die, but some, known as the memory cells, keep circulating in the blood. If the same pathogen tries to attack the body again, these will initiate a rapid immune response. This is known as the secondary immune response.

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