Lecture 9- T cells in infection Flashcards
humoral immunity
product of b cells
involve antibodies
effective against antigens arising from outside cells
eg. vruses toxins and extracellular bacteria
cell mediated immunity
product of t cells do not involve antibodies effective against antigens arising inside cells eg. virus infected cells tumour cells transplanted organs (HLA antigens)
Cytotoxic and helper t cells
All t cells can be identified by CD3. both cell types express T cell receptors for antigen (TCR) which recognise antigenic peptide fragments presented to them by MHC structures. CD8 on the surface of cytotoxic T cells interacts with class 1 MHC determinants. CD4 on the surface of helper T cells interacts with class 2 MHC determinant. T cells can only bind to antigenic epitopes which are presented on the surface of cells by specialised structures call MHC determinants.
responses of t cells
unlike b cells which are produced in bone marrow, t cells come from the thymus. t cell do not make antibodies when they are stimulated by antigens. t cells responses to antigens are of two types;
- cytotoxic responses which have the ability to kill cells that carry recognisable on their surfaces
- helper responses which produce helper cytokines that assist other cells to respond.
- regulator responses which produce inhibitory cytokines (immune hormones).
the major histocompatibility complex-MHC- HlA in humans
the genes coding for MHC proteins on the surface of cells can be divided into class 1 and 2.
class 1 HLA
surface structures found on virtually all nucleated cells except RBC co dominantly expressed polymorphic genes (many alleles in population) present antigenic peptides to CD8 t cells
Class 2 HLA
same as above
surface structures found on specialised antigen presenting cells (APC) and b cells
present antigenic peptides to CD4 t cells
Antigen processing and presentation for class 1 molecules
usually derived from an infectious process or as a result of normal breakdown or normal cell metabolic products within the cell. during an infection with a virus for example, virus specified material is synthesised within the infected cell. some of these viral proteins are broken down within the cell and fragments of them appear on the surface associated with class 1 MHC. these peptide antigens can then be recognised by cytotoxic T cells.
Antigen processing and presentation for class 2 molecules
antigenic material can be taken up by antigen presenting cells (such as macrophages or b cells), broken down into small fragments within the cells, and then peptide fragments presented on the surface by class 2 MHC structures. protein antigens processed in this way will be presented to helper t cells. usually result from the processing of antigens which have been taken up (phagocytes) and degraded by specialised antigen presenting cells.
activation of cytotoxic T cells
with the help of immunological hormones from helper t cells, these activated cytotoxic t cells (with MHC class 1 presented) will divide and differentiate into populations of effector cells which can recognise specific target cells and kill them.
general model of lymphocyte activation
antigen binding to specific receptors on the surface of antigen sensitive b or t cells delivers the first activation signal. the cell then expresses receptors for a growth hormone (cytokine). following receipt of the second activation signal in the form of the appropriate cytokine secreted by an inducer cell, the lymphocyte becomes fully activated and is able to respond by division and differentiation to its immune effector state.
cytokines and their functions
interleukin is a cytokine.
low molecular weight glycoproteins secreted by various cells
regulate the quality, amplitude and duration of immune and inflammatory reactions
produced transiently and locally
can be regarded as immunological hormones
often have multiple, overlapping actions depending on concentration, presence of other cytokines, cell type responding, history of cell
some cytokines also produced by cells other than the blood lineages.
also effect non immune cellular and system behaves
over reactive immunity to innocuous substances
some people generate strong antibody responses of a particular class (IgE antibodies) against relatively innocuous substances such as grass pollens or dust components.
these responses lead to symptoms of allergy such as hay fever and asthma.
immune mediated tissue damage
sometimes the immune response to an infection is so vigorous it causes pathological damage to the host tissues. some infections of hepatitis lead to liver failure through cytotoxic t cells. sometimes antigen-antibody complexes are produced in such high concentrations that they lodge in small vessels, activate the complement system and cause vascular damage
immune destruction by infectious agent
helper t cells are central to the activation and regulation of most immune responses. HIV infects CD4 cells and causes them to becomes targets for cytotoxic t cells attempting to clear infected cells. the result is that CD4 t cells are killed in such large number that t helper cell function is compromised leading to immune compromise and eventually AIDS.
Autoimmunity
begins to treat self antigens as if they were non self antigens. this results in a variety of chronic illnesses.
