What is an Antigen-Presenting Cell? - Definition & Types

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  • 0:03 Antigens & The Immune System
  • 1:02 Antigen Presentation
  • 3:26 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Antigen-presenting cells (APCs) help start the body's immune response against infection by finding and activating T cells. This lesson will discuss how the APCs do this, and how the T cells respond.

Antigens & the Immune System

In order to define an antigen-presenting cell, it's helpful to first define an antigen. An antigen is anything that starts, or generates, an immune response. They're like a signal to the body that something is attacking. Antigens are generally small parts of an invading pathogen, a small microbe that can make you sick. Antigens can be many different types of molecules, including proteins, carbohydrates, and even certain pieces of DNA or RNA.

The immune cells that recognize antigens are called lymphocytes. Lymphocytes are able to recognize and remember antigens, so that at a later time, they can mount a quicker, more efficient attack against the pathogen. B cells can recognize antigens of any type of molecule, and they do not need any help to do so. T cells, on the other hand, can only recognize protein antigens. In addition, they need help to recognize antigens. The help comes in the form of antigen-presenting cells (or APCs).

Antigen Presentation

Some immune cells don't recognize specific antigens like lymphocytes do. Instead, they survey the body, randomly eating up anything that could potentially be harmful. These cells are called phagocytes. Phagocytosis means a cell ingests, or eats, something large. Phagocytes can eat up whole bacteria, small viruses, or parts of dead cells. They help keep the body free of debris, like workers picking up litter on the side of a highway.

There are several different types of phagocytes, but the ones most likely to be APCs are monocytes and dendritic cells (DCs). When a monocyte or DC eats something harmless, it simply digests it, similarly to how your body digests food. However, when they eat something dangerous, signals cause the monocyte or DC to mature. When monocytes mature, they become macrophages; DCs are still called DCs.

The signals that alert the macrophages and DCs of danger also send these cells to areas of the body called lymph nodes. Lymph nodes are small spots where lymphocytes hang out and can meet up with APCs. This helps ensure the APC will find a lymphocyte that recognizes its antigen. If the APC simply kept moving through the blood stream, it might never find its partner among all the other blood cells.

When the APC finally finds a T cell that recognizes its antigen, it presents the antigen to the T cell. This normally activates a type of T cell called a helper T cell that, as the name implies, helps the immune system's response begin. The activated helper T cell replicates itself and causes the body to mount a strong attack against the invading pathogen.

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