BIO 5.8-5.11 - Adaptive immunity

Two distinguishing features of adaptive immunity

• Specificity - the ability to recognise and target specific antigens

• Immunological memory - the ability to remember antigens and mount a larger, faster response when encountering the same antigen again

Antibody

Proteins called immunoglobulins that are produced in response to specific antigens

Functions of antibodies (PIANO)

• Precipitation - antibodies bind to soluble antigens and cause them to precipitate out of solution

• Inflammation - antibodies trigger a release of histamine

• Agglutination - antibodies bind to antigens and form antigen-antibody complexes that clump together to be phagocytosed

• Neutralisation of pathogens - antibodies bind to antigens which are required for pathogens to enter host cells, thus preventing the pathogen from invading

• Neutralisation of bacterial toxins - antibodies bind to toxins and block their action

• Opsonisation - antigen-antibody complexes are formed and tag the pathogen for destruction

Structure of antibodies

• Y shaped molecule made up of 4 polypeptide chains - 2 identical heavy chains and 2 identical light chains

• The variable regions acts as receptors for antigens

Steps of the humoral immune response (5)

1. Clonal selection - An antigen or APC reaches the lymph nodes and binds to a specific B cell that recognises the antigen

2. Helper T cells bind to the antigen and release cytokines that activate B cells and stimulate them to divide

3. Clonal expansion - B cells differentiate and divide into plasma B cells and memory B cells

4. Plasma B cells produce and secrete antibodies that bind to antigens, forming antigen-antibody complexes that are phagocytosed

5. Memory B cells remain in the body after the infection to enable a stronger, faster immune response if the antigen is encountered again

Antigen presenting cells in the humoral immune response

• Dendritic cells and macrophages are APCs that engulf and digest antigens

• These antigens are presented on MHC-II makers, which are recognised by B cells to initiate the humoral immune response

Naive B cells in the humoral immune response

• If the antigen has never been encountered before, a naive B cell will phagocytose the antigen

• The naive B cell edits its DNA so that it becomes a specific B cell to the antigen that it engulfed, allowing it to carry out clonal selection and initiate the humoral immune response

T helper cells (include 3 functions)

• Control the activity of other immune cells

• T helper cells are activated when they bind to antigens presented on APCs

• 1. Release cytokines to stimulate B cells and cytotoxic T cells to divide

• 2. Activate cytotoxic T cells

• 3. Enhance the action of phagocytes

Cytotoxic T cells

• Destroy body cells infected with viruses by releasing cytotoxins such as perforin which punches holes in the cell membrane to induce apoptosis

• Target specific antigens presented on MHC-I markers

• Can also target abnormal cells such as cancer cells or organ transplants

Steps in the cell-mediated adaptive response (5)

1. APCs displaying foreign antigens on MHC-II markers bind to specific helper T cells (usually in lymph nodes)

2. Helper T cells secrete cytokines that stimulate clonal selection and expansion to produce more helper T cells and memory T cells

3. Helper T cells secrete cytokines that activate cytotoxic T cells

4. Cytotoxic T cells bind to body cells presenting abnormal MHC-I markers and release perforin to induce apoptosis

5. Memory T helper cells and memory T cytotoxic cells remain in the body to provide future immunity

Naive cytotoxic T cells in the cell-mediated response

• Naive cytotoxic T cells bind to abnormal MHC-I markers on body cells or MHC-II markers on APCs

• Helper T cells release cytokines that stimulate cytotoxic T cells to proliferate, producing activated cytotoxic T cells and memory T cells through clonal selection and expansion