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15-16 The adaptive immune system increases its response to a pathogen after initial exposure

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  • Adaptive immunity involves humoral immunity and cell-mediated immunity
  • Dendritic cells ingest foreign antigens in the body and present them to the immune cells. They are also important in educating immature T cells and help to maintain the stimulation of B cells so that they continue to make antibodies.

In contrast to the innate immune system, the adaptive immune system's initial response to a pathogen is weak or non-existent, but as the host has time to respond, this system becomes quite vigorous. To respond to a pathogen, the adaptive immune system must learn to recognize it. The mental picture to have in your mind is that of the body taking bits and pieces of the pathogen (its antigens) and presenting them to certain cells of the adaptive immune system. Those cells that can react with the antigen then marshal a bilateral response with one set of cells producing antibodies (humoral immunity) and the other set activating a group of cells to attack the pathogen (cell-mediated immunity). In this part, we will describe the cells that are involved in the processing and response to antigens, followed by the next section when we will put it all together and look at the reaction of the entire immune system to two different types of pathogens.

In addition to their role in innate immunity, phagocytic cells also play a critical role in adaptive immunity. Macrophages and monocytes take a portion of the debris left over from the digestion of a pathogen and provide that as antigens to the adaptive immune system. This ability is not only present in macrophages but B-lymphocytes and dendritic cells as well. These cells are collectively referred to as antigen presenting cells. One of the more important cell types capable of antigen presentation are dendritic cells.

Dendritic cells

Most tissues of the body contain a cell type with long tendrils that wrap around and between neighboring cells. Because these branch-like projections are so similar to the intricate dendrites of nerve cells, these immune system components are called dendritic cells. Dendritic cells, Figure 15-14, arise in the bone marrow and migrate to tissues throughout the body for differentiation. They are found in skin, mucous membranes, bloodstream, lymph and all solid organs of the body except the brain, the eyes and testes.

Figure 15-14 A dendritic cell

A dendritic cell

Dendritic cells are found in most tissues of the body, projecting their branches through out the immediate area, they serve to monitor foreign antigens that may exist. If an antigen if found, it is taken up by the dendritic cell and presented to the immune system.

Three separate classes of dendritic cells play roles in the immune system.

  1. Dendritic cells in various tissues play a pivotal role in activating the immune system. Due to their placement throughout the body and their projections snaking through tissues, they are among the first cells to be exposed to invading pathogens. During the response to a parasite they take up antigens from it and present these to the immune system.
  2. Dendritic cells located in the thymus help to educate immature T cells by exposing them to various antigens (see below).
  3. Dendritic cells maintain the stimulation of B cells so that they continue to synthesize antibodies. Dendritic cells located in various lymphoid tissues carry out this role.
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