Response to Infection

Communicable diseases: are usually caused by micro-organisms, can be transmitted from one person to another

Pathogens

• disease causing organisms

• defence against pathogens can be specific or non-specific

• non-specific a generalised response against all pathogens, infections or injuries

• specific work against selected pathogens

Bacteria: are microscopic living organisms, usually one-celled, that can be found everywhere

• they can be dangerous, such as when they cause infection, or beneficial, as in the process of fermentation (such as in wine) and that of decomposition.

• pathogenic bacteria may enter the body in many ways, such as through the mouth or through cuts in the skin

• they invade the body and are able to multiply. if they multiply sufficiently they can cause an infection

• the infection may be caused by the microbes themselves, or by poisons called toxins that they produce

Viruses

• are 10-100 times smaller than bacteria

• are able to multiply only within the living cells of a host and therefore tend to target specific tissues (cells) in the host

• the ‘flu’ virus will lock onto a target cell in the respiratory system and dock onto the cell surface. they then enter the cell

• once they’re in the cell they use it like a factory; they take it over and make it produce thousands, or millions, of new copies of the virus, which come streaming out of the cell

• they infect other cells to make more viruses, or they escape from the body and infect a new victim

• taking over a cell can kill the host cell which has a consequence for us because it might damage the mucosa (the lining of your airway). this means you get inflammation.

Transmission of Pathogens

• Transmission by contact through direct or indirect means

• Ingestion of contaminated food or drink

• Transfer of bodily fluids

• Airborne transmission

• Infection by droplets

• Transmission by vectors

Three lines of defence

1. Keep microbes out of the body

2. Non-specific attack on microbes

3. Specific attack on microbes

Skin

• Barrier; prevents entry of micro-organisms. Large amounts of bacteria live on skin → occupying bacteria makes it difficult for new (pathogenic) establishments

• Sebum (oily secretion) → substances which kill some pathogenic bacteria

• Sweat → salts and fatty acids prevent growth of many micro-organisms, lysozyme kills bacteria

Mucous Membranes

Hair

Cilia

• found in the respiratory tract

Acids

• found in stomach, vagina and sweat

Lysozyme

• found in tears, saliva, sweat, nasal secretions, tissue fluid.

• enzyme → kills bacteria

Cerumen

Flushing action

• occurs in urine flowing through urethra; tears, sweat, saliva

• cleansing action → prevents bacterial growth/travel through ducts/tracts

Protective reflexes

Sneezing

• Stimulus of irritation of walls of nasal cavity

• Forceful expulsion of air from lungs

Coughing

Vomiting

• Stimulus of psychological; excessive stretching of stomach; bacterial toxins

• Muscles of abdomen and diaphragm contract

• Stomach contents expelled

Diarrhoea

• Stimulus of irritation of small/large intestines by bacteria/virus/protozoans

• Increased peristalsis to quickly remove irritant

• Material spends less time in intestine → faeces watery

2nd Line of Defense

• Fever (elevation of body temperature). Increases production of killer T cells, increases metabolic rate which speeds up tissue repair, increases heart rate/blood flow which enables lymphocytes to reach sites of infection faster, creates a non-optimum temperature that kills pathogens

Specific Defences: Recognises foreign molecules and acts to inactivate or destroy them. If it fails, diseases may occur. Three important aspects: acts against specific pathogens or foreign substances, not restricted to initial infection site, recognises and launches stronger attack on previous encountered pathogen

Antigens: Any substance capable of causing a specific immune response e.g. proteins, carbohydrates, lipids, whole microbes, flagellae, cell walls, virus, cell structures from other individuals, pollen grain, egg white

Self-Antigens: Human cells have many surface proteins, our immune cells do not attack our own proteins. Our cells in another person’s body can trigger an immune response.

Non-Self-Antigens: Materials recognised by the immune system as not belonging to the organism. Stimulate an immune response.

Two types of specific immunity

→ Humoral or antibody mediated (B cells): Involves B-cell production of antibodies that bind antigens resulting in either neutralization, lysis or phagocytosis

• Antibodies: Specialised protein produced in response to a non-self-antigen. Protein group called immunoglobulins (Ig)

• Antigen-Antibody Complex: Specific antibody attaches to specific antigen. Result in destruction/inactivation of the antigen

• Agglutination: The clumping together of microorganisms or of blood cells due to a reaction between substances on the surfaces of the cells and antibodies in the surroundings.

• Humoral response: A macrophage engulfs a pathogen. Macrophages presents the pathogen to a helper T call and B-cell. Helper T cell secretes cytokines which activates the specific B cell to enlarge and divide through mitosis producing clones of themselves. B cell proliferates and differentiate into plasma cells which makes specific antibody for invading antigen/pathogen. Some clones of B cells become memory cells. Memory cells remain to increase immune response when reinfection occurs. Memory cells respond faster to re-exposure to the antigen.

• First exposure to antigen is the primary response. It takes time for B cells to differentiate into plasma cells and then build up antibody levels. Re-exposure response is the secondary response. Memory cells respond much faster.

• All antibodies combine with the antigen for which they are specific. Antibodies may:

  • Combine with bacterial/viral/toxins/enzymes and inhibit them

  • Bind to viruses to prevent cell entry

  • Coat bacteria for easier phagocytosis

  • Cause agglutination of foreign particles

  • Make substances insoluble and hence easier to phagocytose

  
  

  → Cell mediated (T cells): Involves T-cell recognition of abnormal antigens on the surface of host cells (indicating viral infection or tumorigenic change) and the killing of infected cells.

B and T cells are both produced by bone marrow. B cells mature in the bones. T cells mature in the thymus gland

T cells

• One T cell is specific to one antigen

• May be stimulated by cytokines during Humoral immunity

• Intercellular infection of non-self antigen stimulates T cells

• Killer T cell, Helper T cell, Suppressor T cell, Memory T cell

Cell Mediated Response