1. Adaptive Defenses
T Lymphocytes
• T lymphocytes constitute the "cellular" arm
of acquired/specific immunity
• T lymphocytes play a central role in
controlling the acquired immune response
• Serve as crucial effector cells through
antigen specific cytotoxic activity and the
production of soluble mediators called
lymphokines (the cytokines produced by
lymphocytes)
2. Adaptive Defenses
T Lymphocytes
• There are two major subsets of T lymphocytes that
differ in effector function ,MHC restriction and
accessory molecule usage
• helper T cell (CD4+Tcells)
• cytotoxic T cell (CD8+Tcells)
3. Adaptive Defenses
Lymphocyte Functions
• T cells
• secrete lymphokines (cytokines)
• help activate T cells
• cause T cell proliferation
• activate cytotoxic T cells
• stimulate leukocyte production
• stimulate B cells to mature
• activate macrophages
• secrete toxins that kill cells
• secrete growth-inhibiting factors
• secrete interferon
• cellular immune response
4. Adaptive Defenses
cytokines (Lymphokines)
• Cytokines are basically polypeptide
hormones
• The term is often used to denote those
molecules which are the products of cells of
the immune system or which act upon such
cells
• In common with other hormones, cytokines
exert their effects by binding to specific cell-
surface receptors which signal to their
target cells
5. Adaptive Defenses
cytokines (Lymphokines)
• Cytokines act at very low
concentrations
• They are short-lived and may act
locally, either on other cell types
(paracrine) or on the same cell type
(autocrine), or systemically
(endocrine)
6. Adaptive Defenses
Antigen presentation
• T cells must see antigen in the form of
peptides bound to self MHC
molecules
• Class I MHC is adapted for mainly
antigenes produced inside the cells
• Class II MHC is specially adapted to
present extracellular antigens
7. Adaptive Defenses
Antigen presentation
• Three sets of cells can present antigens to
T lymphocytes, which are named Antigen
Presenting Cells (APCs)
• APCs are Dendritic cells, activated
Macrophages or B cells, which have both
clas I and class II MHC
8. Adaptive Defenses
Antigen presentation
• Most real antigen presentation situations
require the binding to a receptor which
triggers endocytosis
• This receptor can be for complement or the
Fc of IgG or it can be an innate,
carbohydrate receptor on
macrophages/dendritic cells .
• In normal situations the critical antigen
presenting cell (APC) is the dendritic cell
9. Adaptive Defenses
Antigen presentation
• The role of dendritic cells is to act as
antigen sensors
• They recognize antigen via their innate
(carbohydrate or complement) receptors
• These receptors can trigger activation
(upregulation of antigen presenting and
accessory functions) and migration from
the peripheral tissue to the lymph node via
the afferent lymphatics
10. Adaptive Defenses
Antigen presentation
• Within the lymph node the antigen
presenting DC can interact with the many T
cells constantly trafficking through .
• Interact with MHC/peptide – TCR
• The principal one which concerns us is the
association of the CD28 molecule on the T
cell with either of 2 ligands B7.1 (CD80) and
B7.2(CD86) molecule on the dendritic cell.
11. Adaptive Defenses
Antigen presentation
• These molecules are termed accessory
molecules and we know that the CD28
molecule delivers an essential second
signal to the T cell without which the T cell
does not become activated (indeed it
becomes unresponsive)
12. CYTOKINES, T CELL SUBSETS
• CD4+ helper T cells are capable of
differentiating from an initial common state
(TH) into 2 apparently distinct types called
TH1 and TH2.
• These subtypes differ in their cytokine
secretion
14. CYTOKINES, T CELL SUBSETS
• The commitment of TH0 cells to
become TH1 or TH2 is influenced by
cytokines secreted by the 2 subtypes
themselves and by macrophages , NK
cells and mast cells as shown in the
following diagram
16. CYTOKINES, T CELL SUBSETS AND Ig
ISOTYPE REGULATION
• The outcome of this differentiation switch
is to activate 2 different pathways of
immunity which are associated with
different antibody isotypes
• The TH1 pathway is essentially cell
mediated immunity, with the activation of
macrophages, NK cells, cytotoxic T cells
and a prolonged inflammatory response
• The cytokines secreted by TH1 cells also
boost production of IgG2a antibody
production in mice
