5.2 Antigen Processing and Presentation

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) that initiate primary T cell responses by presenting antigens to naive T cells in secondary lymphoid organs.

Conventional DCs are the most efficient APCs for initiating primary T cell responses due to their high expression levels of peptide-MHC complexes, costimulators, and cytokines.

DCs are strategically positioned in locations where microbes are likely to enter the body, ready to capture and present their antigens.

Specialized DCs (cDC1) can transfer internalized proteins from phagosomes into the cytosol, a process called cross-presentation, allowing them to efficiently present antigens to CD8+ T cells.

Mature DCs, unlike other APCs, express high levels of peptide-MHC complexes, presenting them to T cells and initiating the immune response.

Dendritic cells (DCs) capture antigens from microbes and transport them to lymph nodes, where they present these antigens to naive T cells, initiating primary T cell responses.

Type 1 cDCs (cDC1) are specialized for cross-presentation, presenting ingested antigens on class I MHC molecules to CD8+ T cells, triggering the activation of cytotoxic T lymphocytes (CTLs).

Plasmacytoid DCs (pDCs) are known for their high production of type I IFN, essential for the innate immune response against viruses.

Type 2 cDCs (cDC2) are responsible for presenting captured antigens to CD4+ T cells, initiating the activation of helper T cells that orchestrate various immune responses.

Monocyte-derived DCs (moDCs) can develop from monocytes under inflammatory conditions, but their roles in immune responses are not yet fully understood.

Conventional DCs are the main cells that capture antigens and transport them to secondary lymphoid organs, where they present these antigens to naive T cells.

Langerhans cells, located in the skin, are one of the earliest identified types of dendritic cells, playing a crucial role in skin immunity.

Dendritic cells (DCs) are the most effective cells for capturing and transporting antigens to naive T cells in lymph nodes, where they initiate T cell activation.

Exposure to microbial products enhances the antigen-presenting function of APCs, making them more effective at activating T cells.

Antigen-presenting cells (APCs), particularly DCs, are crucial for activating naive T cells, starting the adaptive immune response.

Microbes and protein antigens entering through epithelia are concentrated in lymph nodes before being presented to T cells, allowing for efficient immune activation.

The spleen, a secondary lymphoid organ, is the primary site for capturing blood-borne antigens, playing a critical role in immune defense.

MHC molecules bind and display antigens to T cells, allowing these immune cells to recognize and target specific pathogens.

T lymphocytes can only recognize antigens presented by MHC molecules, either class I or class II, on the surface of cells. They cannot recognize soluble, cell-free antigens.

Dendritic cells (DCs) are the most effective antigen-presenting cells (APCs) for activating naive T cells because they are uniquely equipped to capture, process, and present antigens in a way that triggers a robust immune response.

APCs must transport antigens to secondary lymphoid organs, where they encounter naive T cells, enabling efficient immune activation.

Intact folded proteins, particularly those from extracellular microbes, represent the primary source of foreign protein antigens that stimulate T cell-mediated immune responses.

MHC molecules, due to their structure, can only bind and present short peptides derived from larger proteins, making these peptides the primary targets for T cell recognition.

Nonprotein antigens, such as carbohydrates, lipids, and nucleic acids, are typically recognized by antibodies, activating a humoral immune response, rather than by T cells.

APCs provide additional signals, called costimulators, in addition to antigens presented by MHC molecules, to fully activate T cells and initiate an effective immune response.

CD4+ and CD8+ T cells, also known as helper T cells and cytotoxic T lymphocytes (CTLs), respectively, are the types of lymphocytes that recognize peptide antigens displayed by MHC molecules.

Cross-presentation is the process by which DCs can initiate CD8+ T cell responses to the antigens of ingested cells, effectively presenting antigens from infected or cancerous cells to CTLs for destruction.

The class II MHC pathway involves the breakdown of proteins in lysosomes and late endosomes by proteolytic enzymes, generating peptides that are then loaded onto class II MHC molecules, ultimately presenting them to CD4+ helper T cells.

Invariant chain (Ii) is associated with newly synthesized class II MHC molecules and travels from the ER to endosomal vesicles, preventing premature peptide loading and ensuring that the correct peptides bind in endosomes.

Proteins synthesized by extracellular microbes, after being taken up by APCs, are degraded into peptides that bind to class II MHC molecules, ultimately being presented to CD4+ helper T cells.

CD4+ helper T cells, upon activation, orchestrate the elimination of extracellular antigens by activating other immune cells, such as B lymphocytes, which produce antibodies to neutralize the threat.

CD8+ cytotoxic T lymphocytes (CTLs) are specialized to destroy cells infected with intracellular pathogens, such as viruses and some bacteria, by recognizing and killing cells displaying foreign antigens on class I MHC molecules.

Cross-presentation is the process by which DCs capture antigens from virus-infected or tumor cells, process them through the class I MHC pathway, and present them to CD8+ T cells, effectively triggering an immune response against the infected or cancerous cells.

Peptides generated in the cytosol by proteasomes are transported to the endoplasmic reticulum (ER), where they bind to newly synthesized class I MHC molecules, initiating the process of antigen presentation to CD8+ T cells.

TAP, a transporter protein, specializes in moving peptides ranging from 8 to 16 amino acids from the cytosol into the endoplasmic reticulum (ER) for binding to class I MHC molecules, ensuring efficient antigen presentation.

Immunoproteasomes are specialized proteasomes found in immune cells like DCs and other APCs, specifically designed to generate peptides that bind well to class I MHC molecules, enhancing the antigen-presenting capacity of these cells.

Tapasin is a protein that assists in loading peptides onto newly synthesized class I MHC molecules in the endoplasmic reticulum (ER), ensuring efficient and stable peptide binding.

Due to their structural properties, peptides have a higher affinity for class I MHC molecules compared to class II MHC molecules, leading to their preferential binding to class I MHC molecules in the ER.

Peptide antigens stabilize the Class II MHC molecules on the surface of antigen-presenting cells (APCs), ensuring their stability and increasing their ability to present antigens to T cells.

Helper T lymphocytes (CD4+ T cells) activate B lymphocytes, which then differentiate into plasma cells that secrete antibodies specific to the recognized antigen, aiming to eliminate extracellular antigens like bacteria and toxins.

Cytosolic antigens, like viral or tumor-derived proteins, are processed and presented by class I MHC molecules to CD8+ cytotoxic T lymphocytes (CTLs), triggering the killing of cells harboring these intracellular threats.

The CD4 coreceptor on T cells recognizes nonpolymorphic regions on class II MHC molecules, contributing to the stability of the interaction between the T cell receptor (TCR) and the MHC-peptide complex, enhancing antigen recognition.

CD4+ T cells specifically recognize peptides presented by major histocompatibility complex (MHC) molecules on antigen-presenting cells (APCs), crucial for initiating and coordinating the immune response.

CD8+ cytotoxic T lymphocytes (CTLs) play a crucial role in eliminating cells that harbor intracellular pathogens or cancerous cells by directly killing these infected or malignant cells.

Mature DCs, due to their high expression of peptide-MHC complexes, costimulators, and cytokines, create an environment ideal for activating T lymphocytes, triggering a robust and effective immune response.

Specialized DCs (cDC1) can efficiently transfer internalized proteins from phagosomes into the cytosol, a process called cross-presentation, allowing them to present antigens from ingested cells to CD8+ CTLs, initiating a cytotoxic response against these cells.

Activated DCs, in response to chemokines, migrate from epithelia and tissues via lymphatic vessels, transporting captured antigens to lymph nodes, where they present these antigens to naive T cells, initiating the adaptive immune response.

Dendritic cells (DCs) are strategically located at the common sites of entry for microbes and foreign antigens, acting as sentinels to capture and initiate immune responses against these threats.

Chemokines, a type of signaling molecule, play a crucial role in promoting DC migration from epithelia and tissues to lymph nodes, enabling them to transport captured antigens to T cells for immune activation.

All nucleated cells have the capacity to present peptides derived from cytosolic protein antigens on class I MHC molecules to CD8+ cytotoxic T lymphocytes (CTLs), enabling the recognition and destruction of infected or cancerous cells.

Proteins that are present in the cytosol are degraded by proteasomes into peptides that are then transported to the endoplasmic reticulum (ER) and loaded onto class I MHC molecules, allowing for the presentation of these antigens to CD8+ T cells.