Humoral Immunity: T-Dependent, T-Independent Antigens, and Class Switching

00:01 The term T-dependant antigen is used to denote the fact that most B-cell responses require assistance from helper T-cells.

00:10 So an antigen will be taken up by a dendritic cell, it’ll be processed and presented to a naïve T helper cell.

00:19 This T helper cell will then interact with B lymphocytes and cause their proliferation and differentiation into plasma cells.

00:32 These plasma cells will go on and secrete a soluble version of the antibody molecule.

00:40 The interaction of the helper T-cell with the B-cell can be facilitated by the fact that the B-cell itself can act as a professional antigen presenting cell for antigen experienced T helpers.

00:55 In other words, dendritic cells are required to activate naïve T-cells, but once they have been activated by a dendritic cell, the B-cell can take over that function of acting as a professional antigen presenting cell; showing peptides to the T-cell receptor on the helper T-cell.

01:14 However some antigens are T-independent antigens with respect to the B-cell response.

01:22 These are antigens that tend to have repeating structures which extensively cross-link the B-cell receptor or cross-link Pattern Recognition Receptors and the B-cell receptor together.

01:35 But because T-cells do not participate in the response to these T-independent antigens, there is no class switching and only IgM is produced.

01:46 Because in order to get class switching to IgG or IgA or IgE production, you need CD40 ligand on the surface of the T-cell to interact with CD40 on the surface of the B-cell.

01:57 And if there’s no T-cell involved, then that class switching will not occur.

02:02 Examples of T-independent antigens include bacterial lipopolysaccharide, capsular polysaccharides of bacteria and polymeric proteins.

02:14 So here we can see a T-independent antigen with multiple repeating structures on its surface, powerfully cross-linking the B-cell receptor on the surface of the B-lymphocyte.

02:27 This will cause the B-cell to proliferate without any assistance from helper T-cells.

02:33 And those B-cells can differentiate into plasma cells.

02:37 But those plasma cells will only secrete the IgM class of antibody and tiny little bits of IgD antibody as well.

02:45 But they will not class switch to produce IgG, IgA or IgE.

02:51 For T-dependant antigens which is the vast majority of antigens, B-cells can be induced to class switch following interaction with appropriate helper T-cells.

03:03 So here you can see a naïve B-cell co-expressing on its cell surface, IgM and IgD antibodies of identical antigen specificity.

03:13 You can also see on the surface of the B-cell, the MHC Class II peptide and the molecule CD40.

03:21 In the presence of helper T-cells, the CD40 ligand molecule on the surface of the T-cell interacts with the CD40 on the surface of the B-cell.

03:32 This interaction will cause the helper T-cell to release cytokines and this combination of CD40 ligand binding to CD40 on the B-cell together with cytokines will cause the B-cell to class switch.

03:48 In this particular example, it is switched to produce the IgG class of antibody.

03:55 These B-cells will then differentiate into plasma cells and secrete a soluble version of the IgG antibody.