BT302 Final Term Past Papers

Notes For BT302 Final Term · #Topic 85 · CD3 complex Structure & Function

The CD3 stands for Cluster of differentiation 3. This structure is present adjacent to TCR. This is basically the transducing element of TCR. It exists as a complex form which is composed of five following proteins (as shown in fig.9.2), that’s why it is also called as CD3 complex

1. One Gamma(γ)
2. One delta(δ)
3. Two Epsilon (ε)
4. Two Zeta (ζ)

Figure 9.2: The structure of CD3 complex The CD3 complex is considered as invariant proteins means with conserved regions without variability. It has no role in determining the specificity of T-Cell. However, it is necessary for surface expression of TCR during development of T-cell. This has very important role in transducing signals from T-cell surface receptor to interior of cell which is called as Signal transduction. This transduction signals arise after engaging of antigen with TCR in association with MHC complex.

#Topic 86

Cell Surface Molecules involved in T-Cell & Other Interaction

The interface between antigen presenting cell (Target Cell) and T-lymphocyte occur for mounting a strong immune response against antigen. This interaction between TCR & MHC molecules are not so strong and called as immunological synapse. In this immunological synapse, the cell surface molecules on T-cells & their interacting molecule on antigen presenting cells are involved. T-cells also express co-receptor for MHC molecule (e.g CD8 & CD4). Moreover, the expression of supporting molecules are increased by the action of cytokines released from either T-cells or antigen presenting cells. The increased expression of supporting molecules for making immunological synapse strong are called as modulators of immune system. In addition to these molecules, some molecules are also required for T-cell activation. For the activation of T-cells following two signals are required Signal 1 is generated after engagement of TCR with Antigen/MHC complex Signal 2 is generated after engagement of supporting molecules with their corresponding ligands For signal 2, the co-stimulatory molecules are also required for activation of T-cells e.g C28 is a co-stimulatory molecule on T-cell surface & its ligand exists on APC which is known as B7-1. If there is lack of co-stimulation, then there is no activation of T-cells occur and this process is called as cellular anergy. For the full activation of T-cells, the engagement of Immunological synapse is mandatory. As a result the antigen presenting cells (APC) must possess & present peptides to Tcells.

#Topic 87

Accessory Molecules

The interaction between TCR on T-cells & antigen/MHC complex on APC is considered as weak for mounting a strong immune response. In order to stabilize this interaction, there is the need of additional molecules in addition to TCR & antigen/MHC which are called as accessory molecules. These molecules are invariant molecules and have no role in determining the specificity of T-cells. However, these accessory molecules can modulate the immune system in either positive or negative manner. These molecules are critical to success or failure of controlling the immune response to foreign antigen like infectious agents. Moreover, these are also involved in aberrant response to self-antigens like in autoimmune diseases and response to tumors or cancerous cells. These molecules mainly promote or suppress the immune response. The nomenclature of these molecules are according to their function. Followings are the important accessory molecules (Fig 9.3 & 9.4) CD4 This molecule of T-cell is for binding to class II MHC which ensures the binding of T-helper (Th) cells to antigen presenting cells CD8 This accessory molecule is for binding to Class I MHC which ensures the binding of cytotoxic T (Tc) cells to target cell CD2 This accessory molecule on the surface of T-cell has a ligand known as Leucocyte Function Antigen 3 (LFA-3) on antigen presenting cells (APC) Leucocyte Function Antigen 1 (LFA-1) This molecule has ligand known as Intracellular Adhesion Molecule-1 (ICAM-1) CD28 CD28 is the accessory molecule on T-cell and has ligand called as B7-1/B7-2