Introduction – Cell-Mediated Immunity

00:01 We’re going to explore cell-mediated immunity.

00:05 Cell-mediated immunity is immunity that is provided by cytotoxic T-cells and by T-helper cell-mediated activation of macrophages.

00:15 It’s required to defeat intracellular pathogens which are hidden from the effects of antibody and complement.

00:24 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.

00:35 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.

00:51 And that’s where we need cell-mediated immunity.

00:56 So at the core of cell-mediated immunity is the helper T-cell.

01:02 And these act predominantly by producing soluble molecules called cytokines.

01:07 And these cytokines can help activate cytotoxic T-cells or cytotoxic T-lymphocytes (CTL) as we often call them.

01:17 And they can also help activate macrophages.

01:27 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.

01:39 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.

01:51 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.

02:00 Diversity for both of these types of receptors is called-- is created by recombination of T-cell receptor genes.

02:08 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.

02:18 And both of these recombination processes occur within the thymus; αβ T-cells recognize peptides presented by MHC molecules. In contrast, γδ T-cells can either recognize antigens directly, entirely on their own just like antibodies do, or they can recognize lipoprotein and glycolipids that are presented by the MHC-like molecule CD1.