Physical and Chemical Barriers to Infection Week 12

Physical Barriers Skin Mucus (snot!) Elaine Chen’s animation

Chemical barriers Interferon (see later) Interferon Cytokines Group of proteins secreted by immune cells to recruit and activate other immune cells (activates either humoral or cellular response) Complement proteins (see later)

Lymphatic system Distribution of lymphoid organs and tissues which make up the immune system Lymph nodes

Lymphatic system

Blood cells Human blood showing different kinds of blood cells. Various white blood cells (phagocytes) have densely staining nuclei.

White blood cells

White blood cells (phagocytes) Neutrophil cells that have ingested bacteria. The bacteria appear as the smaller purple rod shapes inside the cells Radioactively labelled macrophages and a few lymphocytes

Non-specific vs. Specific responses

Non-specific response Pathogen Invades Tissue Non-specific defences Specific defences (see next section) Barriers to entry Physiological mechanisms Fever Chemical mechanisms Complement proteins Interferon Phagocytes and natural killer (NK) cells Inflammation Basophils Mast cells and Platelets Histamine, Phagocytosis

First line of defence Chemicals on the body surface also inhibit infective organisms The first line of defence against infection is the body surface which acts as a barrier (physical/chemical barriers – already discussed)

Second line of defence Includes: Fever Interferon/Cytokines Complement proteins Phagocytes (white blood cells) Natural killer (NK) cells Inflammation Blood clotting

Fever Fever

Interferon/ Cytokines Interferon/cytokines

Complement proteins Complement proteins lyse many bacterial species. This attracts phagocytes to the site of infection. Bacteria which have been coated by other complement proteins are readily ingested by the phagocytes. Complement proteins

Phagocytosis Stages of phagocytosis: a neutrophil ingesting a bacterium Macrophage animation

Natural Killer (NK) cells 3 animations

Inflammation a) Injury to an otherwise healthy skin b) Vasodilation and increased permeability c) Phagocyte migration from capillaries to cut area d) Phagocytosis of bacteria and other debris by macrophages Inflammation occurs if bacteria enter a cut

Inflammation Pathogens enter tissues Mast cells Basophils Platelets Produce Histamine and other substances Blood vessels dilate Increased blood flow to the region Capillaries become permeable and leaky Complement proteins attract phagocytes Phagocytes move to the area Redness Heat Edema Pus Increased Phagocytes

Blood Clotting Red blood cells trapped in protein fibres Some steps involved in blood clotting and wound healing