INTRODUCTION TO HOST RESPONSEa Th 2 -cell response. In mice infected with S. mansonithe egg granuloma formation is a T
cell-dependent hypersensitivity reaction mediated by T helper cells (CD4+Th cells)
which are specifically primed for schistosome egg antigen. As the infection proceeds there
is a gradual reduction in the size of the granuloma formed around continuously incom-
ing eggs. This may be due to the macrophages failing to stimulate the Th 1 cells which
subsequently become unresponsive to antigen-presenting cells. This process has been
described as immunomodulation and may reflect a state of unresponsiveness of a Th 1 sub-
population of CD4+T cells which are soluble extract of antigen-specific. Hence during the
course of a S. mansoniinfection there appears to be both Th 1 and Th 2 cell activity. The
Th 1 cells play a major role in launching the formation of egg granulomas in the early stages
of infection at a time when the granuloma size is maximal. Not only do the Th 1 cells medi-
ate granuloma size and formation but they also enhance the granulomagenic potential of
Th 2 cells, also enhancing antibody production and eosinophilia. During the acute phase,
however, Th 1 cells become unresponsive to stimulation by a new population of antigen-
presenting cells. The result is a decreased cytokine production and cell activation, lead-
ing to diminished granuloma size (immunomodulation). The antigen-presenting cells
(granulate macrophages or accessory cells) are thought to preferentially downregulate Th 1
cells, leaving the Th 2 cells to sustain antibody production, eosinophilia and reduced
granuloma formation.
The T helper cells require two signals in order to carry out their functions. Firstly a
primary signal from the complex of MHC class II molecules and antigen-derived peptide
to induce mitotic activity, producing a clone of a specific group of T helper cells.
Secondly co-stimulatory signals on the antigen-presenting cells are equally necessary for
T cell activation.
n SUMMARY
All vertebrates have the ability to try and protect themselves against invasion by foreign
and non-self material, a response referred to as the immune response. There are two basic
types of response: the innate and the adaptive immune response. Innate immunity is non-
specific and is dependent upon the inherent capability of having phagocytic cells that can
recognise non-self molecular patterns on the membranes of invasive pathogens. If the pathogen
is not destroyed then the infection becomes chronic and an adaptive immune response
develops. This involves the cloning of lymphocytes that have been in contact with the
antigen peptides via an antigen-presenting cell. Those lymphocytes are cloned and retain
a memory of the antigen’s molecular configuration. They subsequently produce specific
antibodies and cytotoxic cells to try and control the infection.
The white blood cells and other phagocytic cells are all participants. The cells originate
in the bone marrow and are subsequently processed in the thymus. Within the spleen
and lymph nodes the lymphocytes make contact with the antigen-presenting cells and the
adaptive immune response develops. T helper cells, mast cells and cytokines all play an
essential role in the adaptive immune response.End of chapter questions
INTRODUCTION TO HOST RESPONSE
Question 5.1 In multicellular organisms, how does the genotype of each individual cell
help with identification of self?