The Immune System Learning objective Describe the role of the T cells (T lymphocytes) in cell-mediated immunity Describe the role of B cells (B lymphocytes) in humoral immunity

Immunity “Free from burden” The ability of an organism to recognize and defend itself against specific pathogens or antigens

Human Body What are antigens? Every cell has unique proteins on its surface called antigens. The antigens on the microorganisms which get into your body are different to the ones on your own cells. Your immune system recognises them as different and produces antibodies as part of the body’s defence.

Antigens Antigens as part of the cell–surface membrane of invading cells = antigen pathogen

The Immune system is the third line of defense against infection Nonspecific defence mechanisms Specific defence mechanisms (Immune system) First line of defence Second line of defence Third line of defence Physical barriers (skin) PhagocytosisCell-mediated response (T Lymphocytes) Humoral response (B lymphocytes)

Lymphocyte production Stem cells in the bone marrow and foetal liver give rise to T lymphocytes (T cells) and B lymphocytes (B cells)

B Cells B cells mature in the bone in the shaft of the long bones (e.g. The femur). The migrate from there to the lymphatic organs (the lymph nodes and the spleen).

T Cells T cells leave the bone marrow and collect in the thymus gland.

T Cells The thymus gland is located above the heart on the windpipe. It is a large in infants, but regresses with age. Immature T cells move from the bone marrow to the thymus to mature before migrating to other lymphatic organs

Cell-mediated immunity Involves T cells which defend against: Intracellular bacteria and viruses Protozoa, fungi, flatworms and roundworms Cancerous cells and transplanted foreign tissue

Cell-mediated immunity T cells are activated when: Phagocytes that have engulfed and broken down a pathogen present some of the pathogens antigens on their cell-surface membrane, Body cells invaded by viruses present viral antigens on their cell surface membrane, Cancer cells present antigens on their cell surface membrane.

Cell-mediated immunity These are termed antigen-presenting cells because they present antigens of other cells on their cell- surface membrane. As T cells will only respond to antigens attached to a body cell, this type of response is called cell-mediated immunity.

Cell-mediated immunity

Cytotoxic T cells T cells kill body cells infected by pathogens. They produce a protein that makes holes in the cell-surface membrane, so that the cell becomes freely permeable to substances and dies as a result. Most effective against viruses. As viruses need living cells to reproduce, this stops them from multiplying.

Humoral immunity Humoral immunity is so named because it is associated with the serum (non-cellular part of the blood), involving substances found in the ‘humours’, or extracellular body fluids.

Humoral immunity It involves B cells secreting antibodies which recognise and bind to antigens and protect us against circulating viruses, and bacteria and their toxins (free antigens). As many as 10 million different types of B cell develop in each of us and each one recognises one specific antigen.

Humoral immunity

Plasma cells These secrete antibodies specific to the antigen that stimulated their development Each plasma cell lives for only a few days, but can produce 2000 antibodies per second

Memory cells These are retained in the lymph nodes to provide future immunity. In the event of a second infection, B memory cells react more quickly and vigorously than the initial B cell reaction.

Antigenic variability Diseases like chickenpox and measles are of a single type and easily recognised. Influenza viruses have over 100 different strains. This is known as antigenic variability.