Lymph: fluid in the tissue spaces that carries protein molecules and other substances back to the blood
Lymphatic vessels permit only one-way movement of lymph
Lymphatic capillaries are tiny, blind-ended tubes distributed in tissue spaces. They are:
Microscopic in size
One cell thick
“Leaky”: They have a poor “fit” between adjacent cells, which results in higher wall porosity than is found in capillaries
Swelling (edema) of tissues caused by blockage of lymph vessels
Lymphangitis—inflammation of lymph vessels, may progress to septicemia (blood infection)
Elephantiasis—severe lymphedema of limbs resulting from parasite infestation of lymph vessels
(filariasis)

Functions include defense and white blood cell formation
Filter lymph of bacteria, cancer cells, and damaged tissue cells; prevents them from entering blood circulation; helps to localize infection
Located in clusters along the pathway of lymphatic vessels
Lymphoid tissue organ located in the mediastinum
Plays a vital and central role in immunity
Produces T-lymphocytes or T Cells (Helper T Cells and Cytotoxic (killer) T Cells)
Composed of three masses of lymphoid tissue around the openings of the mouth and throat
Palatine tonsils (“the tonsils”)
Pharyngeal tonsils (adenoids)
Lingual tonsils
Provide protection against bacterial infection through the nasal-oral cavities
Subject to chronic infection and enlargement

Largest lymphoid organ in the body
Functions include phagocytosis of bacteria, foreign substances, and old RBCs; recovers iron; acts as a blood reservoir (contains up to 500ml of blood)
Protects the body from pathogenic bacteria, foreign tissue cells, and cancerous cells
Made up of organs, specialized cells and molecules
Provides immunity (resistance) to the “enemies” of the body
The immune systems comes into play when our external defenses fail to prevent infection
In addition to its qualities as a strong, flexible, and waterproof “shield,” the skin also acts as a chemical barrier to infection
Glands secrete antimicrobial substances that can be produced in greater quantities as needed
These broad spectrum antimicrobials are aimed primarily at bacteria and fungi, but some have antiviral properties as well
A breach of the skin leads to an inflammatory response intended to increase the attention of the immune system at the source of entry
This strategy serves to localize infection, if possible

Given that a wide variety of microbes are airborne, it is not surprising that the respiratory tract is an important point of entry for pathogens
Mucous in the nose and pharynx can trap larger particles that can be expelled by coughing or sneezing
In the lower bronchial tree, the muco-ciliary elevator moves mucous toward the pharynx, where it is expelled through coughing or spitting, or is swallowed
Substances in the alveolar spaces cause aggregation of microbes, facilitating their destruction
Antimicrobials, some like those found in the skin, are found in the respiratory tract
Together, these defences yield sterile alveoli under normal circumstances
Food and water contamination are significant sources of infection
Untreated water and poor food-handling or cooking practices are responsible for a wide range of illnesses
Saliva contains lysozyme, an enzyme that can damage the cell walls of some bacteria and fungi
The high acidity of gastric juices are a first line of defence against infection through the oral consumption of pathogens
The lining of the stomach protects it; the lack of such a lining in the oesophagus makes the vomiting reflex -- which can also serve to clear pathogens -- both hazardous and uncomfortable
At least one bacterium, H. Pylori, has adapted to the acidic environment of the stomach, and is blamed for up to 80% of all ulcers of the stomach and duodenum
Bile salts may also be antibacterial, and antimicrobial substances are also found in the gut
The gut is home to specialized antibodies, and friendly flora abound; this may serve to keep pathogenic bacteria in check
The fecal-oral route is an important pathway of infection for a number of pathogens

The urinary tract is flushed of bacteria through voiding
This can be impaired in the case of prostatic hypertrophy
In females, the urethra is relatively short, and therefore more vulnerable to ascending infection
The kidneys produce an antibacterial peptide that can aid in controlling the extent of bacterial infection
Similar antibacterial substances are produced in the vagina, cervix, and uterus
These appear to be regulated by hormones
The vagina is mildly acidic, which would also lend some protection from infection
Tears serve to flush particles – including microbes – from the eyes
Tears contain the antibacterial and antifungal enzyme lysozyme
Phagocytes
These cells of non-specific immunity ingest and destroy foreign cells or other harmful substances via phagocytosis

Specific immunity provides protection against specific bacteria, viruses or toxins
Involves the ability of the body to recognize, respond to, and remember harmful substances or bacteria
On first infection, disease symptoms may occur; upon subsequent infection, the disease agent is rapidly destroyed, and no serious symptoms can be expected
Inherited or inborn immunity: immunity to certain diseases from birth
Acquired immunity
Natural immunity—exposure to causative agent is not deliberate
Active—active disease produces immunity
Passive—immunity passes from mother to fetus through placenta or from mother to child through mother’s milk
Artificial immunity—exposure to causative agent is deliberate
Active—vaccination results in immunity
Passive—protective material developed in another individual’s immune system and given to previously non-immune individual

Antibodies
Protein compounds with specific combining sites
Combining sites attach antibodies to specific antigens (foreign proteins), forming an antigenantibody complex
Antigen-antibody complexes may:
Neutralize toxins
Agglutinate (clump) foreign cells
Promote phagocytosis
Initiate the complement cascade (which increases rates of phagocytosis or causes cell lysis through osmosis)
Most numerous of immune system cells
They are responsible for antibody production
They “wander” in the tissues and fluids of the body, “seeking out enemy cells”
They are most dense in lymph nodes and other lymphatic tissues
Two major types: B Cells and T Cells
Immature B Cells are formed from stem cells, in the liver and bone marrow before birth, and in the bone marrow exclusively after birth; they “mature” when they come into contact with a specific antigen
Activated B cells develop into plasma cells; plasma cells secrete antibodies into the blood; circulating antibodies provide immunity

T Cells are formed from stem cells originating in bone marrow that undergo maturation in the thymus
T Cells become “sensitized” when they encounter a specific antigen
T Cells kill invading cells by releasing a substance that poisons cells and also by releasing chemicals that attract and activate macrophages to kill cells by phagocytosis
When lymphocytes encounter ‘their’ antigen, they proliferate through cell division
This clonal selection allows for individuals to develop immune systems that are adapted to their environment as well as their own immunological history
Once an infection is eliminated, lymphocytes die off, but we retain “memory cells” that reflect our prior experience of infection, and allow us to mount a more rapid and vigorous response to pathogens if we encounter them in the future
B Cells appear to maintain memory for many decades after exposure, even in the absence of the antigen
Proliferation yields more of them that are antigen-specific
They become plasma cells more quickly
They make more powerful antibody more rapidly
They produce antibody with a higher affinity
T Cells appear to maintain memory for many years, but it is not understood if this memory persists in the absence of the antigen
Memory T Cells proliferate more rapidly than naïve T Cells
Longer-term memory is derived from live rather than dead vaccines
Oral vaccines are more effective for pathogens of the gut