Blood Components and Immunity

• Blood consists of red blood cells, white blood cells, platelets, and plasma.
• Plasma makes up over half the volume of blood.
• Red blood cells make up the majority of the other half of blood.
• The remaining fraction consists of white blood cells and platelets.

Components of Blood

• Red blood cells are biconcave discs without a nucleus to maximize haemoglobin capacity.
• White blood cells are large cells with a large nucleus and different structures/functions depending on the type.
• Platelets are fragments of cells.
• Plasma is a clear, straw-colored aqueous solution.

Plasma

• It is a straw-colored liquid in which other blood components are suspended.
• Plasma is important for transporting substances including:
• Carbon dioxide (waste product of respiration) dissolved in the plasma is transported from respiring cells to the lungs.
• Digested food and mineral ions (dissolved particles absorbed from the small intestine) are delivered to cells around the body.
• Urea (waste substance) dissolved in the plasma is transported to the kidneys.
• Hormones (chemical messengers) released into the blood from endocrine organs (glands) are delivered to target tissues/organs.
• Heat energy (created in respiration) is transferred to cooler body parts or the skin where heat is lost.

Red Blood Cells

• Red blood cells are specialized cells that carry oxygen to respiring cells.
• They are full of haemoglobin, which binds to oxygen to form oxyhaemoglobin.
• Haemoglobin + oxygen => oxyhaemoglobin.
• Red blood cells lack a nucleus, allowing more space for haemoglobin.
• A red blood cell's biconcave disc shape gives it a large surface area to volume ratio for maximizing oxygen diffusion.

White Blood Cells

• White blood cells are part of the body's immune system.
• Specialized cells defend against pathogenic microorganisms.
• There are two main types of white blood cells: phagocytes and lymphocytes.

Phagocytes

• Phagocytes perform phagocytosis to ingest pathogens:
• They have a sensitive cell surface membrane to detect chemicals produced by pathogenic cells.
• Once they encounter a pathogenic cell, they engulf it and release digestive enzymes to digest it.
• Phagocytosis is a non-specific immune response.

Lymphocytes

• Lymphocytes produce antibodies:
• Antibodies are proteins with a specific shape that is complementary to antigens on a pathogen's surface.
• Lymphocytes provide a specific immune response, as the antibodies produced only fit one type of antigen on a pathogen.
• Lymphocytes produce antibodies that are specific to the antigen on the pathogen.

The Body's Immune System

• The body's immune system is highly complex, and white blood cells are its main component.
• The immune system prevents infectious organisms from reproducing and destroys them after a pathogen enters the body.
• An organism has immunity when it has sufficient antibodies to protect it from a particular disease, thus it does not suffer from the disease or its symptoms.

Response to Infection

• The stages of infection and subsequent immune response are:

1. The pathogen enters the bloodstream and multiplies.
2. A release of toxins (in the case of bacteria) and infection of body cells causes symptoms.
3. Phagocytes recognize the pathogen, engulf, and digest it (non-specific response).
4. The pathogen encounters a lymphocyte that recognizes its antigens.
5. The lymphocyte starts producing specific antibodies to combat that pathogen.
6. The lymphocyte clones itself to produce lots of lymphocytes, all producing the required specific antibody.
7. Antibodies destroy the pathogens.
8. Phagocytes engulf and digest the destroyed pathogens.

Antigen vs. Antibody

• An antigen is a molecule found on the surface of a cell.
• An antibody is a protein made by lymphocytes that is complementary to an antigen. When attached, antibodies clump antigens together and signal the cells they are on for destruction.

Vaccines

• Vaccines induce immunity to infectious diseases and have drastically reduced or eradicated certain diseases worldwide, including smallpox, measles, mumps, and tetanus.
• A vaccine contains harmless versions of a pathogen and scientists ensure vaccines contain harmless pathogens, such as: Killing the pathogen Making the pathogen unable to grow or divide (attenuated vaccine) Using fragments of pathogens, rather than whole cells
• A vaccine may be administered orally, nasally, or via injection.

How Vaccines Work

• The antigens within a vaccine trigger an immune response once in the bloodstream: Lymphocytes recognize the antigens in the bloodstream. Activated lymphocytes produce antibodies specific to the antigen encountered. Memory cells are produced from the lymphocytes. Memory cells and antibodies subsequently remain circulating in the bloodstream. Future infection by the same pathogen will trigger a response that is much faster and larger compared to the initial response. The rapid nature of the response prevents the pathogen from causing disease, and the individual is then considered immune

Platelets

• Platelets are fragments of cells involved in blood clotting and forming scabs.
  • Platelets arrive to stop the bleeding when skin is broken (i.e. there is a wound).
  • A series of reactions occur within the blood plasma:
  • Platelets release chemicals that cause soluble fibrinogen proteins to convert into insoluble fibrin.
  • This forms an insoluble mesh across the wound.
  • Red blood cells become trapped, forming a clot.
  • The clot dries and develops into a scab.
• This process helps prevent excessive blood loss and protects the wound from bacteria until new skin has formed.