The document discusses the functions of antigen-presenting cells (APCs), specifically focusing on dendritic cells, macrophages, and B cells in their roles in the immune response. It details how macrophages excel in degradation and presentation of particulate antigens, while dendritic cells are critical for uptake and activation of T cells. Additionally, it explains the mechanisms of antigen processing and presentation by these cells, illustrating their importance in both innate and adaptive immunity.

1. Assignment
Submitted by:
M.Irtaza abbas
Department:
BS MLT
Semester:
6th
(B)
Roll no:
42
Submitted to:
Mam Anam Tariq
Subject:
Basic Immunology
Minhaj University Lahore

2. Q1: Which of the following 3 types of antigen
presenting cells (APCs) is specialized for degradation
and presentation of particulate antigens to T cells?
Discuss.
1. dendritic cells
2. Macrophages
3. B cells
Among these 3 types of antigen presenting cells (APCs) Macrophages
are specialized for degradation and presentation of particulate antigens to
T cell because they have high phagocytic activity than other APCs like
dendritic cell which also have phagocytic activity but less than
macrophages.B cells which are also Antigen presenting cells have no
phagocytic activity.B cells can internalize antigen that binds to their B
cell receptorand present it to helper T cells.
Antigen presenting cells (APCs):
Antigen-presenting cells (APCs) are a heterogeneous group of immune cells that
mediate the cellular immune response by processing and presenting antigens for
recognition by certain lymphocytes such as T cells. Classical APCs include dendritic
cells, macrophages, Langerhans cells and B cells.
Difference betweendendritic cells , macrophages andB cellas APCs:
Feature Dendritic cells Macrophages B cells
Immune response
Innate immunity Innate immunity Adaptive immunity
Specific antigen receptors No No Yes
Location Skin and mucosal
epithelium
(Langerhans
cells), lymphoid
tissue, connective
tissue
Lymphoid tissue,
connective tissue,
body cavities
Blood, lymphoid
tissue
Antigen type intracellular
antigens and
extracellular
antigens
extracellular
antigens
extracellular
antigens
MHC molecule associated
with antigen presentation
Class I MHC and
class II MHC
Class II MHC Class II MHC
Co-stimulation High level B7
expression
Low level B7
expression,
induced by
bacteria/cytokines
No B7 expression
unless induced upon
activation by Th
cells

3. Mechanismof Antigen presenting cells (APCs):
An antigen-presenting cell (APC) is an immune cell that detects, engulfs, and informs
the adaptive immune response about an infection. When a pathogen is detected, these
APCs will phagocytose the pathogen and digest it to form many different fragments
of the antigen. Antigen fragments will then be transported to the surface of the APC,
where they will serve as an indicator to other immune cells.After phagocytosis by
APCs, the phagocytic vesicle fuses with an intracellular lysosome forming
phagolysosome. Within the phagolysosome, the components are broken down into
fragments; the fragments are then loaded onto MHC class I or MHC class II
molecules and are transported to the cell surface for antigen presentation.T
lymphocytes cannot properly respond to the antigen unless it is processed and
embedded in an MHC II molecule. APCs express MHC on their surfaces, and when
combined with a foreign antigen, these complexes signal a “non-self” invader. Once
the fragment of antigen is embedded in the MHC II molecule, the immune cell can
respond. Helper T- cells are one of the main lymphocytes that respond to
antigen-presenting cells. Recall that all other nucleated cells of the body expressed
MHC I molecules, which signal “healthy” or “normal.”
Mode of actionof Macrophages:
A macrophage is a cell of the innate immune system that engulfs and digests
pathogens, and then presents fragments on its surface as a signal. Such signals are
picked up by other cells of the adaptive immune system, hence antigen-presenting
cell.Macrophages participate in both innate and adaptive immune responses. As part
of innate immunity, macrophages are activated by antigens that are bound to PAMP,
Toll-like, scavenger, and mannose receptors. In an adaptive response, antigens are
recognized using receptors for intermediary molecules, such as antibody and
complement fragments.Macrophages interact with T cells in order to bring about T
cell activation in target organs, and are themselves activated by inflammatory
messenger molecules (cytokines) produced by the T cells. Macrophages produce toxic
chemicals, such as nitric oxide, that can kill surrounding cells.

4. Figure 1. An APC, such as a macrophage, engulfs and digests a foreign bacterium. An
antigen from the bacterium is presented on the cell surface in conjunction with an
MHC II molecule Lymphocytes of the adaptive immune response interact with
antigen-embedded MHC II molecules to mature into functional immune cells.
Q2: Which of the following 3 types of antigen
presenting cells (APCs) is critical in uptake and
presentation of antigen to T cells? Discuss
1. dendritic cells
2. Macrophages
3. B cells
Among these 3 types of antigen presenting cells (APCs) Dendritic cells
are most critical in uptake and presentation of antigens to T cells.They
play a very important role in activation of T cells.Macrophage which is
also a Antigen presenting cell has a difference with dendritic cell.This
difference is that macrophages have high phagocytic activity but low
antigen processing and presentation activity but on the other hand
dendritic cells have high antigen processing and presenting activity but
low phagocytic activity.
Mode of actionof dendritic cells:
Dendritic cells are the most efficient antigen-presenting cells. They take
up antigens and pathogens, generate MHC-peptide complexes, migrate from the sites
of antigen acquisition to secondary lymphoid organs and, finally, they physically
interact with and stimulate T lymphocytes.Indeed, dendritic cells are the only
antigen-presenting cells that induce the activation of resting T cells, both in vitro and
in vivo. Thus, dendritic cells initiate adaptive immune responses and determine
tolerance. To do so, dendritic cells have developed unique membrane transport
pathways.The main function of these innate cells is to capture, process, and present
antigens to adaptive immune cells and mediate their polarization into effector
cells.Naive T cells leave the thymus and enter secondary lymphoid organs. In

5. secondary lymphoid organs, naïve T cells are activated by mature dendritic cells. T
cell activation requires 2 signals: TCR and costimulation.The T cell encounters
a dendritic cell (DC) bearing its cognate peptide in an MHC molecule, and binds the
peptide-MHC though CD3 and CD4 or 8. Subsequently, co-stimulation occurs
through DC-bound CD86, CD80, OX40L and 4-1BBL. This induces
full activation and effector function in the T cell.
Explanation of diagram:
After a dendritic cell recognizes and attaches to a pathogen cell, the pathogen is
internalized by phagocytosis and is initially contained within a phagosome.
Lysosomes containing antimicrobial enzymes and chemicals fuse with the phagosome
to create a phagolysosome, where degradation of the pathogen for antigen processing
begins. Proteases (protein-degrading) are especially important in antigen processing
because only protein antigen epitopes are presented to T cells by MHC II.
Summary:
 Antigen-presenting cells break down large-molecular-weight antigens into 10 to
30 amino acid fragments for loading onto HLA class I and II molecules.
 Antigen-presenting cells can be either “professional” or “amateur” cells.
 Dendritic cell subsets are uniquely suited for antigen presentation.
 Antigen-presenting cells are involved in both the innate and adaptive immune
responses.
 Macrophages and B cells ingest antigens by different mechanisms, but both cells
process antigen using the endocytic pathway.
 The endocytic pathway is complex and involves proteolytic enzymes and HLA
class II stabilizing proteins.