Cells, tissues and organs of the immune system

•       Cells of the immune system
• tissues of the immune system
• organs of the immune system

  Cells of the immune system

        leukocytes

Mast cells

Granulocytes

Basophils,

Eosinophils,

        Neutrophils

Monocytes

Macrophages

Dendritic cells

Lymphocytes

T- Lymphocytes

        B- Lymphocytes

NK cell

leukocytes

• The cells responsible for both innate defenses and adaptive immunity are the leukocytes (Greek leukos, white, and kytos, cell).
• All leukocytes originate from pluripotent stem cells in the fetal liver and in the bone marrow.
• Some Pluripotent stem cells differentiate into hematopoietic precursor cells that are destined to become blood cells.
• When stimulated to undergo further development, some leukocytes become residents within tissues, where they respond to local trauma.
• These cells may sound the alarm that signals invasion by foreign org’s.
• Other leukocytes circulate in body fluids and are recruited to the sites of infection after the alarm has been raised.

• The average adult has approximately 7,400 leukocytes per cubic millimeter of blood.
• This average value change during an immune response.
• For example, in most infections the white blood cell (WBC) count may increase as leukocytes migrate from the blood to the site of invasion.
• The loss of the leukocytes from the blood (leukopenia) and specific signals from responding cells (colony stimulating factors) result in the maturation and release of new cells from the bone marrow.
• A transient increase in blood-borne leukocytes (leukophilia, also called leukocytosis) may result until the infection subsides.
•        For leukocytes to respond to pathogens, they must detect their presence.

• Many leukocytes detect repeating chemical patterns found in microbial macromolecules.
• These unique microbial signatures are known as pathogen-associated molecular patterns (PAMPs).
• The PAMPs form from specific sequences within
• proteins, polysaccharides, lipids, and nucleic acids (i.e., components of the microbial macromolecules, such as lipopolysaccharide [LPS], peptidoglycan, fungal cell wall components, viral nucleic acids, and other microbial structures).
• PAMPs inform the host to microbial presence, sounding the alarm of potential infection.

•   The receptors that bind to PAMPs are both soluble and membrane bound; they are called pattern recognition receptors (PRRs).

       1. soluble PRRs includes,

• acute-phase proteins,
• mannose-binding protein (of the lectin complement pathway), and
• C-reactive protein (CRP).

     2.The membrane-bound PRRs are found on phagocytic cells, enabling them to

   (1) distinguish between potentially harmful microbes and other host molecules,

   (2) respond by ingesting and degrading the source of the PAMP.

The different types of leukocytes, including those that detect PAMPs are.

1. Mast cells
2. Granulocytes
3. Basophils,
4. Eosinophils,
5. Neutrophils
6. Monocytes
7. Macrophages
8. Dendritic cells

9. Lymphocytes 

10.  T- Lymphocytes

11. B- Lymphocytes
12. NK cells

1. Mast Cells

• Mast cells are bone marrow-derived cells that differentiate in

connective tissue.

• indented nucleus and a cytoplasm filled with specialized organelles called

granules.

• Mast cells are nonphagocytic, but when stimulated, they rapidly release the contents of their granules into the extracellular environment, a process called degranulation.
• Mast cell granules contain histamine, prostaglandins, serotonin,

heparin,dopamine, platelet-activating factor, and leukotrienes.

• these compounds influence the tone and diameter of blood vessels, they are

termed vasoactive mediators.

• mast cells (and basophils) possess high-affinity receptors for the type of antibody associated with allergic responses.
• When they become coated with this type of antibody, binding of antigen to the antibody triggers the release of the preformed vasoactive mediators-play a major role in certain allergic responses such as eczema, hay fever,and asthma

   2. granulocytes

3.Monocytes

• Monocytes are mononuclear leukocytes with an ovoid- or kidney-shaped nucleus and granules in the cytoplasm.
• They are produced in the bone marrow
• enter the blood,
• circulate for about 8 hours,
• enlarge,
• Migrate into tissues
• and mature into macrophages or dendritic cells

   Macrophages 

• Macrophages are mononuclear phagocytic leukocytes,

they are larger than monocytes,

• contain more organelles that are critical for phagocytosis, and
• have a plasma membrane covered with microvilli.
• Macrophages are similar to dendritic cells in their ability to bridge innate resistance and adaptive immunity.
• They have PRRs that recognize specific PAMPs, and
• binding of their PRRs induces phagocytosis.
• They also migrate as activated cells to lymphoid tissues to present antigen to lymphocytes.
• Antigen presentation by macrophages is similar to that of dendritic cells, with the exception that macrophages do not appear to stimulate naive lymphocytes but rather stimulate T and B cells that have been previously sensitized to antigens (i.e., memory cells).
• Macrophages spread throughout the host and take up residence in specific tissues,
• where they are sometimes referred to as "fixed" macrophages

Dendritic cells

• Dendritic cells are not a single cell type; they are a heterogeneous group of cells so named because of their dendrite (neuron)-like appendages.
• They arise from various hematopoietic cell lineages;
• both lymphoid and myeloid dendritic cells are known.
• constitute about 0.2% of peripheral blood leukocytes and can be stimulated to

mature by specific cytokines.

• Most dendritic cells, however, are tissue-bound, where they play an important

role in bridging innate resistance and adaptive immunity.

• They are present in the skin and mucous membranes of the nose, lungs, and

intestines, where they constantly survey their environment.

• Dendritic cells are highly specialized cells that are programmed to detect

and phagocytose foreign substances, especially pathogens.

• Dendritic cells have numerous membrane-bound receptors, including (PRRs).
• The end product of phagocytosis is the proteolytic degradation of the ingested particles.
• Once degraded, the resulting remnants of phagocytosis (antigens) are futher

processed so as to be displayed on the dendritic cell surface.

• Dendritic cells then migrate to lymphoid tissues to "present" the antigens to lymphocytes, sharing vital information about invaders and stimulating the specific immune response.
• In presenting antigen, dendritic cells are capable of eliciting specific immune responses from naive (i.e., never encountered a specific antigen) T cells.
• Dendritic cells can also capture foreign materials with their cell surface

receptors for antibodies and complement.

• both antibody and complement proteins can coat microorganisms or other foreign materials (the process of opsonization), enhancing their phagocytosis .
• These degraded products can also be presented to lymphocytes as antigens.
• dendritic cells destroy invading pathogens as part of the innate resistance process, they also help trigger specific immune responses.

Lymphocytes

 tissues and organs of the immune system