The Immune system

Biology II: Form and Function

The Immune System• Nonspecific defense mechanisms

– Physical barriers (skin, mucous membranes– Internal defense 1

• Phagocytic neutrophils• Macrophage monocytes• Natural killer cells• The antimicrobial protein complement

• Specific defense mechanisms (the Immune system)– T- and B-Lymphocytes, antibodies

Physical barriers• Integument

– is acidic (pH 3-5)– Flakes away, taking away particles– Subcutaneous adipose tissue helps cushion

physical contact– Sweat contain lysozyme

• Mucous – prevents microbes from reaching sensitive

epithelial tissue– Washes surface of dust– Viscous natures traps particles (cilia)

Nonspecific defenses: the front line– Monocytes are transformed into macrophages that

ingest microbes and fuse them with lysosomes (highly oxidative)

– Neutrophils can also act as macrophages, but more commonly release poisons that work on a large scale

– Natural killer cells destroy already infected cells by releasing molecules of perforin that breach the target cell. Sudden influx of water lyses foreign cell

– The protein complement system aggregates to form membrane attack complexes also to breach target cells, as well as releasing proteins that inhibit virus transcription (also interferons)

The inflammatory/temperature response

– Invasion of foreign material causes injured cells to release chemical alarm signals (e.g. histamine)

– Chemical alarm signals promote vasodilation– Vasodilation and increased permeability of capillaries

causes edema (tissue swelling)– Increased permeability allows macrophages to cross

over into infected site– Macrophages release interleukin-1, causing body to

raise temperature (fever), which causes mild anemia– Localized infections can be serious enough to cause

systemic response

The Immune System– The immune system responds to the presence of antigens, specific complex protein molecules associated with bacteria, viruses and other foreign bodies

– Humoral immune response produces antibodies (B-lymphocytes)

– Cell-mediated immunity (T-lymphocytes)– Specific defense against microbial invasion: response

elicited by antigens• Specificity: Specialized lymphocytes produce antibodies• Diversity (over 109 different types of antibody)• Memory (acquired/passive immunity)• Self/nonself recognition (defense is specific against antigen.

Failure to recognize healthy, non-antigen tissue causes auto immune diseases (e.g. lupus))

Humoral Response/Antibodies

– Produced by B-Lymphocytes (from bone marrow)– Antibodies belong to a group of proteins called

Immunoglobulins (Ig)– General structure is “Y” shaped, made of two light

and two heavy polypeptide chains– Heavy chains specify type of antibody (IgM, IgG,

IgA, etc.)– Light chains responsible for antigen specificity

• Initial encounter between B-lymphocyte with specific antibody and antigen causes clonal selection: division of B-cells to– Memory cells– Plasma cells that produce more antibodies

that flag antigen for destruction by • aggregation of protein complement system• Macrophage activity

Cell-mediated Response

• T-cells have similar receptor sites to B-cells

• Response to binding between recognition site and antigen is production of:– Cytotoxic T-cells (Tc) attack cells labeled for

destruction by helper T-cells

– Helper T-cells (Th), produce cytokines (interleukin) to regulate immune response

When things go wrong (1)– Acquired Immuno-Deficiency Syndrome

(AIDS) is represented by a breakdown of the immune response system, caused by the virus HIV (Human-Immunodeficiency Virus)

• HIV infects T-cells until entire population of certain types are destroyed

• Infected T-cells secrete chemical that block defense by other T-cells

• HIV blocks recognition sites so that infected cells cannot be destroyed by immune system

When things go wrong (2)– Antigen shifting causes acquired immunity

to fails by changing the ‘face’ of the antigen• Seen in Malaria, Bilharzia

– Autoimmune diseases caused by failure of immune system to recognize self (causes inflammation and organ damage)

– Allergies caused by abnormal response of • B-cells to allergen (immediate hypersensitivity),

producing a histamine reaction• T-cells (delayed hypersensitivity)