BIO 5.8-5.11 - Adaptive immunity + Allergy

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 (6)

1. An antigen enters the body and may be engulfed by an APC

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

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

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

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

6. 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, cytotoxic 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

Allergic reaction

A rapid and vigorous overreaction of the immune system to a previously encountered allergen

Steps of the allergic response (6)

1. Allergen binds to B cells

2. B cells produce IgE antibodies

3. IgE binds to mast cells

4. When the allergen is encountered again, it binds to the activated mast cells via the IgE antibodies

5. When the allergen bridges the gap between 2 antibodies it triggers the release of histamine

6. Histamine causes blood vessels to dilate and become more permeable, increasing the inflammatory response. It also contracts the muscles of the airways which causes difficulty breathing and increases mucus production.