CELL MEDIATED IMMUNE RESPONSE Flashcards
what are the phases of cell-mediated immunity?
- recognition of cell-associated microbes by T cells.
- activation of T lymphocytes
- clonal expansion and differentiation
- effector functions.
explain recognition of cell-associated microbes by T cells.
Both TH and TC cells can recognize antigen ONLY when it has been processed and presented on the cell membrane of an Antigen Presenting Cell (APC) or Target Cell, respectively in association with self MHC.
Antigen presenting cells : the cells which present antigen to CD4 TH
cells.
Target cells: The cells which present antigen to CD8 TC cells.
MHC is also known as human leukocyte antigen.
This process is called “Antigen processing and presenting”.
what does “processing” and “presenting” means?
Processing= protein antigen is degraded into peptides.
Presentation= association of peptide with MHC and transportation of
MHC/peptide complex to the cell membrane.
what are the Cells which have been induced to function as APCs?
i) macrophages
ii) B lymphocytes
iii) dendritic cells
the differences between T cytotoxic cell and T helper cell?
Tc - immediately kill the cell
Th - produce cytokine
explain activation of T lymphocytes.
Antigen recognition by TCR triggers signals that are delivered to the interior of the cells.
The signals are delivered by CD3 and ζ (zeta) proteins, as well as, the co-receptors (CD4 or CD8).
Co-receptors recognize class II or class I MHC molecules respectively.
In response to antigen, T lymphocytes especially CD4 TH cells,
rapidly secrete several different cytokines that have diverse activities.
The first cytokine to be produced within 1 to 2 hours after activation
is interleukin 2 (IL-2) - main action is to stimulate the proliferation of T cells.
explain clonal expansion and differentiation.
Within 1 or 2 days after activation, T lymphocytes begin to proliferate resulting in expansion of antigen specific clones.
Some differentiate into effector cells that function to eradicate infections.
CD4 TH cells may differentiate into diverse effector cells that
produce distinct sets of cytokines that perform different functions,
such as:
- produce interferon-γ which activate phagocytes, and stimulate the
production of opsonizing & complement binding antibodies. - produce IL-2 which stimulate the production of IgE.
- produce IL-5 which stimulates eosinophils which function mainly
in defence against helminths.
CD8 TC cells _____ and
may require help from_____ to ____ into______.
CD8 TC cells recognize peptides of intracellular protein antigens and
may require help from CD4 TH cells to differentiate into effector
CTLs.
explain effector functions.
There are two types of cell-mediated immune reactions:
i) CD4 TH cells activate macrophages
ii) Effector CTLs kill cells harbouring microbes in their cytoplasm.
explain “Effector CTLs kill cells harbouring microbes in their cytoplasm.”
CTLs induce cell death via two mechanisms :
a) the perforin/granzyme pathway.
b) Fas/FasL pathway
explain “CD4 TH cells activate macrophages”.
- to kill ingested microbes that are able to survive in the vesicles of
the phagocytes - produce substances including reactive oxygen intermediates, nitric
oxide and lysosomal enzymes that kill ingested microbes - produce cytokines that induce inflammation.
explain the perforin/granzyme pathway.
The primary events in CTL-mediated death in the perforin/granzyme
pathway are:
i) Conjugate formation (binding of the CTL to the MHC-peptide).
ii)CTL cytoplasmic rearrangement- golgi stacks and granules in CTL
relocate toward the point of contact with target cells.
iii) Granules contents are released by exocytosis.
iv) Perforin contact with target cell membrane will form pores.
v) Granzyme enters via pore, fragmentation of DNA occurs.
vi) CTL dissociation and recycling.
explain Fas/FasL pathway.
Fas is a transmembrane protein that can deliver a death signal when
cross-linked by its natural ligand – Fas ligand which are found on the
membrane of CTLs.
Interaction of FasL with Fas on a target cell triggers apoptosis.
