Role of Antigen Presenting Cells in Activating the Immune System

Antigen processing by antigen presentingcells (APCs)

~ After phagocytosis of foreign bodies, APCsprocess the foreign proteins (proteaosome orlysosome) and then move these molecules to thesurface with major histocompatibility – MHC class II receptors

Antigen presenting cells (APCs) are specialized immune cells that play a key role in activating the immune system. The process of antigen processing by APCs involves the uptake, intracellular processing, and presentation of antigen peptides to T cells. There are several types of APCs, including dendritic cells, macrophages, and B cells.

1. Uptake: The first step in antigen processing by APCs is the uptake of antigen by endocytosis or phagocytosis. This can occur in various ways, such as through scavenger receptors on macrophages, complement receptor-mediated phagocytosis, or through receptor-mediated endocytosis by dendritic cells.

2. Intracellular processing: After uptake, the antigen is broken down into smaller peptides by intracellular proteases and other enzymes. The peptides are then loaded onto major histocompatibility complex (MHC) molecules within the endoplasmic reticulum (ER).

3. Presentation: Once the antigen peptides are loaded onto MHC molecules, the MHC-peptide complex is transported to the surface of the APC where it can be recognized by T cells. This process is known as antigen presentation.

There are two types of MHC molecules, class I and class II, which present antigen peptides to different types of T cells. Class I MHC molecules present peptides to CD8+ T cells, which are important in recognizing and killing infected or tumour cells. Class II MHC molecules present peptides to CD4+ T cells, which help to coordinate the immune response through cytokine production and activation of other immune cells.

In summary, antigen presenting cells play a critical role in shaping the immune response through the process of antigen processing. By taking up, processing, and presenting antigens to T cells, these cells help to activate the immune system and fight off infections and diseases.

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