Introduction – Cell-Mediated Immunity

We’re going to explore cell-mediated immunity. Cell-mediated immunity is immunity that is provided by cytotoxic T-cells and by T-helper cell-mediated activation of macrophages. It’s required to defeat intracellular pathogens which are hidden from the effects of antibody and complement. So if you have an organism outside of a cell, such as a bacterium or even a virus before it enters a cell, antibody and complement are great at dealing with that particular infection. However, once organisms enter into cells as all viruses do for some of their life cycle, and also some parasites and some bacteria also spend at least some time inside cells; antibody and complement just can’t get at them. And that’s where we need cell-mediated immunity. So at the core of cell-mediated immunity is the helper T-cell. And these act predominantly by producing soluble molecules called cytokines. And these cytokines can help activate cytotoxic T-cells or cytotoxic T-lymphocytes (CTL) as we often call them. And they can also help activate macrophages. Let’s compare the two types of T-cell that have different T-cell receptors; in other words, the alpha-beta (αβ) T-cell and the gamma-delta (γδ) T-cell. So here you can see the αβ T-cell with the T-cell receptor on its cell surface composed of a T-cell receptor α-chain and a T-cell receptor β-chain. And then on the right hand side, you can see a γδ T-cell with its T-cell receptor composed of a γ-chain and a δ-chain. Diversity for both of these types of receptors is called-- is created by recombination of T-cell receptor genes. The T-cell receptor genes for the α-chain and the β-chain in the case of the αβ T-cell receptor, and the γ-chain and δ-chain in the case of the γδ T-cells. And both of these recombination processes occur within the thymus; αβ T-cells recognize...