Chapter 1

Basic Concepts

in Immunology

Lymphocytes encounter and respond to antigen in the peripheral lymphoid organs (PLOs)

3 major types of PLO

- Spleen (blood)

- Lymph node (tissue)

- Mucosal associated lymphoid tissues (epithelia)

PLOs are specialized to trap DC, which

displays Ag to lymphocyte

Circulating lymphocytes

Naïve lymphocytes

Effector cells

Draining lymph nodes

Figure 1-10 Lymphoid tissues

trapping Ag ingested by DC or MØ

presenting Ag to migratory lymphocytes

inducing adaptive immune responses

providing signals to lymphocytes to survive or

apoptosis (maintaining correct # of lymphocytes)

Figure 1-8 part 1 of 2

The organization of a lymph node

Figure 1-8 part 2 of 2

Light micrograph of a section through a lymph node

Figure 1-9

The organization of the lymphoid tissue of the spleen

Figure 1-9 part 2 of 3

The arrangement of discrete areas of white pulp around central arterioles

Figure 1-9 part 3 of 3

Light micrograph of a transverse section of white pulp of human spleen immunostained for mature B cells

PFZ=perifollicular zone

MZ=marginal zone

Co=follicular B-cell corona

GC=germinal center

PALS=periarteriolar lymphocyte sheath

Peyer’s patches

Interaction with other cells as well as with

antigen is necessary for lymphocyte activation

Antigen binding to lymphocyte receptors

Co-stimulatory interactions

Two signals are required for lymphocyte activation

The 3 types of antigen presenting cells

Lymphocytes proliferate in

response to antigen in

peripheral lymphoid organs,

generating effector cells and

immunological memory

Time course of a typical antibody response to antigen

Antibodies deal with extracellular forms of

pathogens and their toxic products

Different pathogens have their own distinct lifestyle

B cells recognize Ags present outside the cells

T cells detect Ags generated inside infected cells

Effector mechanisms of adaptive immunity

Figure 1-24 Antibodies can participate in host defenses in three main ways:

humoral immunity

Neutralization

Opsonization

Complement activation

T cells are needed to control intracellular

pathogens and to activate B-cell responses to

most antigens

Cell-mediated immune responses

CD8 T cells – cytotoxic T cells

CD4 T cells – helper T cells

2 major types of CD4 effector T cells

- Th1 and Th2

Cytotoxic T cells destroy virally infected cells, thereby preventing the spread of virus

Influenza virus budding from an infected cell

A virally infected cell surrounded by cytotoxic T lymphocytes

Activated macrophages destroy mycobacteria that infect them

CD4 and CD8 T cells recognize peptides bound

to two different classes of MHC molecules

MHC class I vs MHC class II

Co-receptor CD8 binds MHC class I

Co-receptor CD4 binds MHC class II

Major Histocompatibility Complex (MHC) Molecules Bind Antigenic Peptides

MHC class I molecules present peptides

synthesized within the cell

MHC class I molecules carry antigenic

peptides from the ER to the cell surface

MHC class II molecules present internalized protein antigens, not endogenously synthesized antigens

Macrophages

B cells

Figure 1-31 Helper T cells, upon activation by MHC class

II+peptide on APCs, in turn activate those

cells, thereby resulting in antigen removal

Defects in immune system result in increased

susceptibility to infection

- immunodeficiency diseases, AIDS

Understanding adaptive immune responses is

important for control of allergies, autoimmune

disease and organ graft rejection

- immunosuppressive drugs

Figure 1-32

Immune responses can be beneficial or harmful,

depending on the nature of the antigen and the genetic

background of the responding individual

3 other successful vaccination campaigns

Vaccination is the most

effective means of

controlling infectious

diseases