Innate and adaptive immunity

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Now that we have discussed the first line of defence, we will be moving on the cellular mechanisms that the body employs to protect against infection. If the first line fails, i.e. if the skin is broken, a pathogen may successfully enter the body and cause infection. In stems the innate immune system, which forms the second barrier to infection.

Adaptive immunity

In acquired (adaptive or specific) immunity, lymphocytes (B cells and T cells) encounter an invader, learn how to attack it, and remember the specific invader so that they can attack it even more efficiently the next time they encounter it. Acquired immunity takes time to develop after the initial encounter with a new invader because the lymphocytes must adapt to it. However, thereafter, response is quick. B cells and T cells work together to destroy invaders. Some of these cells do not directly destroy invaders but instead enable other white blood cells to recognize and destroy invaders.


Innate (natural) immunity does not require a previous encounter with a micro-organism or other invader to work effectively. It responds to invaders immediately, without needing to learn to recognize them. Mechanisms that fall into innate immunity include phagocytosis, interferons, complement, inflammation and fever. Several types of white blood cells are involved:


  • Phagocytes ingest invaders. Phagocytes include macrophages, neutrophils, monocytes, and dendritic cells.
  • Antigen-presenting cells (APC’s) help T cells (T lymphocytes) recognize invaders. Antigen-presenting cells consist of dendritic cells (the most effective), macrophages, and B cells.
  • Some white blood cells release substances involved in inflammation and allergic reactions, such as histamine. Some of these cells often act on their own to destroy invaders.
  • Natural killer cells are formed ready to recognise and kill cancer cells and cells that are infected with certain viruses.



Inflammation is in response to the release of histamine from mast cells. Have you ever noticed how your skin gets red, swollen, and hot when you get hurt, and how sometimes a wound is sensitive to touch?

The purpose of inflammation is to protect the damaged tissue or site of infection, to promote healing, and clear/prevent further infection.

  • Inflammation causes an increase blood flow to a damaged area or site of infection, to get more blood to the site of infection to deliver nutrients and to clear off any waste materials.
  • There is an increase in capillary permeability to make it easier for immune cells to go to the site of infection and fight off any pathogens.
  • The temperature of the infection site is increased to deliver more energy to the cells that need to heal, making the area hot to the touch.
  • Inflammation could trigger the activation of complement to assist in killing off pathogens.


  • Protective proteins such as interferons that are released by white blood cells that recruit more cells of the immune system (i.e. phagocytes and NK cells) to attack or assist in fighting off the pathogen. Interferons assist in the prevention and spread of viral infections.



  • Complement pathway is a non-specific pathway that triggers the opsonization and killing of pathogens. In the complement pathway, certain proteins bind together to form a membrane attack complex that damages the membranes of bacteria and cells (i.e. in worm infections)



Fever is when the thermal set-point of the body is increased. Humans have a basal temperature of between 36.1ºC and 37.2ºC, and during fever this temperature increases (up to 40ºC). Pathogens that have the ability to induce fever are known as pyrogens or pyrogenic. The purpose of fever is to:

  • Increase the energy that damaged cells need to heal themselves, and
  • Inhibit the further spread of pathogens by making the body too hot for them to proliferate easily

Most bacteria thrive at 37ºC, and making the body unliveable for them is one of the main benefits of fever.

Innate Immunity