Keywords: lymphocytes, immunity, antigen, humoral, response Lesson Objectives: Describe the different forms of immunity in the body. (AO1: D) Describe what is meant by ‘phagocytosis’ and how it acts as a defence mechanism. (AO2: C) Explain the role of lysosomes in phagocytosis. (AO1: A) Keywords: lymphocytes, immunity, antigen, humoral, response

One of these things is not like the others….. What do these pictures have in common? AO1 One of these things is not like the others…..

Barriers that help prevent the entry of pathogens into the body: Barriers to Entry AO1 Barriers that help prevent the entry of pathogens into the body: A protective covering - the skin. Epithelia covered in mucus - In the lungs pathogens are trapped by the mucus and pushed back in the trachea by the goblet cells into the stomach. Hydrochloric acid in stomach - low pH destroys the enzymes of the bacteria.

There are two main types of white blood cell that protect your bodies from pathogens.

What are the main defence mechanisms of the body? AO1 What are the main defence mechanisms of the body? Defence mechanisms Non-specific Response is immediate and the same for all pathogens Specific Response is slower and specific to each pathogen Physical Barrier E.g. Skin Phagocytosis Cell-mediate response T Lymphocytes Humoral response B Lymphocytes

Phagocytosis AO1 Phagocytosis – In Greek phagein means ‘to devour’, kytos means ‘cell’ and osis means ‘process’ Bacteria are far too large to cross a cell membrane by diffusion or active transport. So they have to be engulfed by cells in the form of vesicles (fluid-filled cavity). This process is called phagocytosis . This type of white blood cell carried is called a phagocyte. Phagocytosis causes inflammation at the site of infection. This swollen are contains a mixture of dead pathogens and phagocytes, this is pus. The inflammation is caused by the release of histamine. Histamine causes dilation of the blood vessels, increasing the flow of blood, increasing the number of phagocytes to the area.

Phagocytosis AO1

The phagocyte attaches itself to the surface of the pathogen. Phagocytosis AO1 Chemical products of the pathogen attract the phagocyte to move towards it. The phagocyte attaches itself to the surface of the pathogen. The phagocyte engulfs the pathogen to form a vesicle, known as a phagosome. Lysosomes move towards the vesicle and fuse with it. Enzymes within the lysosome break down the pathogen. The process is the same as the digestion of food in the intestine, namely hydrolysis of insoluble molecules into smaller soluble ones. The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm everything else is discharged.

Questions What is a lysosome and what is inside it? Phagocytosis AO2/3 Questions What is a lysosome and what is inside it? An enzyme called lysozyme breaks down the cell walls of bacteria. Among other places, it is found in tears. Suggest a reason why this is so. What purpose does the phagosome have?

Lymphocytes AO1 Lymphocytes are already present in the body, approximately 10million different types. Lymphocytes should be able to differentiate between own body cells and foreign bodies. So ‘antigens’, proteins on the cell-surface membrane help the lymphocytes to differentiate between self and foreign cells.

Lymphocytes AO1 Immune responses that are specific to individual forms of infection are slower to act at first but provide long term immunity. This uses a type of white blood cell called a lymphocyte and there are two types: B lymphocytes (B cells) are associated with humoral immunity (will be covered in lesson). T lymphocytes (T cells) are associated with cell-mediated immunity.

Cell-mediated immunity Causes and symptoms AO1 Cell-mediated immunity T lymphocytes are the white blood cell that responds to foreign or infected cells. This includes bacteria, viruses and cancer cells as well as transplanted tissues as they are genetically different. T lymphocytes can distinguish these cells from your own cells because: Phagocytes that have engulfed and broken down a pathogen present some of the pathogens antigens on their own cell-surface membrane. body cells invaded by a virus also manage to present some of the viral antigens on their cell- surface membrane as a sign of distress. Cancer cells likewise present antigens on their surface. These are called antigen-presenting cells as they present the antigens of other cells on their cell-surface membrane.

Causes and symptoms AO1 As T lymphocytes will only respond to antigens that are attached to a body cell (rather than those in body fluids or on a bacteria), this type of response is called cell-mediated immunity. The stages in the response of T lymphocytes to a infection are as follows:

Pathogens invade body cells are taken in by phagocytosis. The body cell or phagocyte presents antigens from the pathogen on its cell-surface membrane. Receptors (proteins) on certain T helper cells fit exactly onto these antigens. This activates other T cells to divide rapidly by mitosis and form a clone. The cloned T cells: Develop into T memory cells that enable a rapid response to future infections from the same pathogen. Stimulate phagocytes to engulf pathogens by phagocytosis Stimulate B lymphocytes to divide. Kill infected cells

Causes and symptoms AO1 Types of T cell T Helper cells release cytokines which attract new macrophages to the infected area and stimulate phagocytosis as well as stimulating B cell division. T killer cells destroy virally infected cells and cancer cells by crating a protein that causes holes in the infected cells membrane. This makes the membrane freely preamble and the cell dies. This process is called lysis. T memory cells are clones of the original T lymphocyte that live in the body for many years and are present in great numbers causing a quick response to secondary infection by the same pathogen.