LO: Describe the structure of antibodies and their mode of action

Immunoglobulins

• Secreted by B lymphocytes and are called antibodies

Antibodies

• Antibodies and T cell receptors (TCR) have variable (V) and constant (C) regions
• Variable regions binde antigens
• Constant region define function

Function

• Neutralisation
• Opsonisation

Isotypes

• Different constant regions mediate different functions

Immunoglobulin M (IgM)

• First serum antibody produced after infection
• Pentamer with 10 antigen binding sites
• Strongly activates complement cascade
• Neutralises pathogens
• Clumps (agglutinates) microbes
• High IgM serum levels indicate acute or recent infection

Immunoglobulin G (IgG)

• Most abundant Ig in human serum - 80%
• Main mediator of secondary immune response
• Moves across placental barrier and protects foetus
• Passive humoural immunity during early infancy
• High IgG indicates chronic infections or autoimmune diseases

Immunoglobulin A (IgA)

• Two IgA molecules linked by a joining protein
• Immune defence at mucosal surfaces
• Secreted across epithelial cells
• Traps and neutralises pathogens before they enter tissues

Immunoglobulin E (IgE)

• Defence against parasites
• Mediation of allergies
• Main bound to Fc$ϵ$RI receptors on tissue-resident mast cells, basophils or eosinophils

Isotype switching

• VDJ gene rearrangements allow B cells to make different antibodies
• Once right antibody is made, a second gene rearrangement i nthe constant region takes place
• Allows different antibody types to be made from the same gene

LO: Describe the mechanism of B cell maturation, activation and differentiation and the concept of immunoglobulin gene rearrangement

• Immunoglobulin (Ig) genes change during immune development
• These genes have many repeated regions which code for the variable domain
• Variable (V), diversity (D), joining (J), constant (C)
• Random VDJ rearrangement generates many different antibodies from a single immunoglobulin gene
• T cell receptor genes also rearrange
• All cells have immunoglobulin and TCR genes in chromosomes but Ig only rearranges in B cells and TCR in T cells
• Ig and TCR genes onyl rearrange once and on one chromosome so the cell only produces one specific antigen receptor (allelic exclusion)

LO: Discuss the function of B cells and how they contribute to the adaptive immune response

Adaptive immunity

• Gene rearrangement is key
• Gene rearrangement only in B and T cells
• Allows an immune response
• Allows immune system to adapt

Plasma and memory cells

• Memory B cells retain the membrane bound B cell receptor and can keep communications with T cells
• Circulate quietly for decades
• Allows immune system a fast, specific defence upon re-exposure to pathogen
• Plasma B cells remove the membrane anchor from the receptor and secret Ig

LO: Describe the mechanism of T cell maturation, activation and differentiation

LO: Discuss the function of T cells and how they contribute to the adaptive immune response

T cells

• T cells do not bind antigens directly
• Recognise antigens presented by MHC on the cell surface
• T and B cell need to agree on the same antigen to activate clonal expansion
• Prevents the production of self-reactive antibodies and autoimmune disease

Cytokines

• Determine the antibody type made by the B cell

LO: Discuss the theory of clonal selection and the role of effector cells in cell mediated and humoural responses

Clonal expansion

• Stimulated by antigen binding
• Cells that do not bind Ag will die
• B cells need T cell help to produce antibodies