adaptive immunity - week 4

antigen

Any molecule recognized by an antibody or T cell receptor – usually a protein or polysaccharide on a microbe

• One microbe carries many antigens, and each antigen has many epitopes – this is why adaptive responses can be so
  specific

epitope

The small part of an antigen that is actually bound – also called the antigenic determinant

immunogen

An antigen that can trigger an immune response on its own (not every antigen can)

adaptive immunity

a specific response against infection by a particular pathogen

adaptive immunity has two arms:

• humeral immunity - mediated by antibodies secreted by B cells

• cell-mediated immunity - carries out by T lymphocytes

specificity and memory

• Adaptive responses are highly specific – not only to a particular pathogen, but to individual parts of it.

• A unique feature is immunological memory – a faster, stronger response on re-exposure.

B cells and T cells

• Adaptive immunity relies on two cell types: B lymphocytes (B cells) and T lymphocytes (T cells)

• B cells mature in the bone marrow and produce antibodies (immunoglobulins) – humoral immunity.

• T cells mature in the thymus and destroy cells infected with intracellular pathogens – cell-mediated immunity.
  
  

antibodies (immunoglobulins)

• Humoral immunity involves B cells and antibodies (immunoglobulins) – glycoproteins found in blood and tissue fluids.

• Five classes: IgG, IgM, IgA, IgE and IgD, defined by the constant region of the heavy chain.

• Structure: four chains – two identical heavy (H) and two identical light (L) – held together by disulphide bonds

what antibodies do: neutralisation

• neutralisation

• humoral immunity fights pathogens outside cells, in three mains ways

  • neutralisation: antibodies bind pathogens or toxins and stop them entering or acting on cells

What Antibodies Do: Opsonisation and Complement

• Opsonisation: antibody Fab regions bind the pathogen; the Fc region binds receptors on neutrophils and macrophages, “tagging” it for phagocytosis

• Complement activation: bound antibodies trigger the classical pathway – boosting opsonisation, recruiting phagocytes and forming the membrane-attack complex

Primary and Secondary Antibody Responses

B cell receptors

• B cells express immunoglobulin molecules
  in their surface membrane

• These surface immunoglobulins are the B
  cell receptors (BCRs) for antigen

• Each has two identical antigen-binding
  sites, formed by the variable domains of
  one heavy and one light chain

cell-mediated immunity

• Pathogens that have entered host cells are largely protected from antibodies.

• Cellular immunity targets and eliminates these intracellular pathogens through T lymphocytes.

• T cells begin developing in the red bone marrow, then travel to the thymus to mature

CD4 and CD8 T Cells

• T cells express cluster of differentiation (CD) molecules – surface glycoproteins used to identify types of white blood cell.

• CD4 is found on helper and regulatory T cells; CD8 is found on cytotoxic T cells.

three classes of T cells

There are three main classes of T cells:

• Helper (CD4+): activate macrophages,
  dendritic cells and B cells, and support
  cytotoxic T cells, by secreting cytokines

• Cytotoxic (CD8+): kill infected host cells by
  inducing apoptosis (programmed cell death)

• Regulatory (CD4+): suppress helper T cells,
  cytotoxic T cells and dendritic cells

T Cell Activation: Antigen Presentation

How T cells are activated

• T cells cannot recognise pathogens without help.

• Antigen-presenting cells (APCs), such as dendritic cells and macrophages, engulf and digest pathogens into antigen fragments.

• These fragments are displayed on the APC surface on MHC molecules

The Major Histocompatibility Complex (MHC)

• The major histocompatibility complex (MHC) is a cluster of genes whose products help the immune system tell self from non-self.

• MHC molecules bind peptide antigens processed by APCs and present them to the T cell receptor

MHC Class I and Class II

• Class I MHC: on nearly all nucleated cells; presents endogenous peptides (e.g. viral proteins made inside the cell) to CD8+ T cells.

• Class II MHC: mainly on APCs (macrophages, dendritic cells, B cells); presents exogenous peptides (taken up from outside) to CD4+ T cells.

MHC Class I → CD8, MHC Class II → CD4

• MHC class II molecules bind peptides that are
  recognised by CD4 T cells

• MHC class I molecules bind peptides that are
  recognised by CD8 T cells

types of acquired immunity

active vs passive

innate vs adaptive immunity