Chapter 34: Immunity to Infections, CD Molecules, and Cytokines

IMMUNITY TO INFECTION

Human beings are exposed to various microorganisms like:

1. Bacterial
2. Viral
3. Fungal

Besides, some of these are pathogenic organisms. After entering into the body, they cause infection, and our immune system becomes active to get rid of these pathogenic microorganisms.

Non-Specific or Non-Immunological Host Defence Mechanism

These are as follows:

1. Surface defense.
2. Specific or Acquired Immunity.
3. Physiological Means.
4. Others.
5. Immunization.

SURFACE DEFENCE

Physical

The following are the barrier to entry of the pathogenic microorganism:

1. Intact mucous membranes and intact skin will prevent the entry of Micro-organism.
2. Coughing and sneezing will expel out microorganisms.
3. Epithelial cell turnover also prevents the settling of the microorganism.

Chemical

Various chemicals give us immunity like:

1. Gastric acid (low pH) is a local disinfectant and retards salmonella and cholera vibrio growth.
2. Skin lipids produce an acidic medium (low pH) so as not suitable for bacteria growth.
3. Vaginal secretion inhibits bacterial growth.
4. Lysozyme and interferon present in tears, saliva, and nasal secretion attack bacterial cell wall, and lysozyme are bactericidal.
5. Spermin prostatic secretion inhibits the growth of gram-positive bacteria.

Humoral

These are various humoral factors present in the serum-like:

1. The alternative complement pathway is bactericidal.
2. Acute-phase proteins like CRP increase 1000 times in response to pyogenic infection. It binds to bacteria. Other acute phase proteins are α1-antitrypsin, β2-macroglobulin, fibrinogen, ceruloplasmin, and factor B.

Body Surface Bacteria

These body-surface bacteria, which are normally present, may produce the natural antibody or compete with pathogenic bacteria for nutrition.

They also produce antibacterial substances like anaerobic bacteria in the intestine produce a fatty acid, which will retard bacterial growth. Another example is colicin and acid.

SPECIFIC OR ACQUIRED IMMUNITY

This will take place through

1. B-lymphocytes            =          Ab-mediated.
2. T-lymphocytes            =          Cell-mediated.

Ab-Mediated (B-L) Methods

1. Opsonization by IgG or IgM.
2. Neutralization of toxin by IgG.
3. Lysis of bacteria by IgG & IgM.
4. ADCC (Antibody-Dependent Cellular Cytotoxicity).

T-Cell Mediated Methods

1. By CD8⁺ cells—Cytotoxic injury.
2. By CD4⁺ cells—this may act as effectors cells give cell-mediated immunity.
3. Lymphocytes may produce lymphokines.

Complement Mediated Immunity

The complement may be activated by bacteria or other pathogens and leads to the lysis of targets.

PHYSIOLOGICAL MEANS

1. Body temperature: Some bacteria have poor growth at 37Co.
2. Oxygen-Tension: This is high in the lung, and it will inhibit the growth of anaerobes.
3. Hormonal imbalance, like the raised levels of corticosteroids, has an anti-inflammatory effect.
4. Age: Young and old aged are more prone to infection.

OTHERS

1. Species: Some of the species are resistant to certain diseases like leprosy is not seen in animals.
2. Racial: Sickle cell patients are immune to malarial infection. This racial immunity is absolute in plants.
3. Individuals: Tuberculosis in twins has 85% chances if they are monovular while 25% are binovular.

IMMUNIZATION

This is an artificial process where an individual is rendered immune.

Active Immunization

It makes a non-immune individual acquires a long-lasting ability to respond to an organism or its toxic products by generating its own protective mechanisms.

Active immunization may be:-

1. Natural acquired by infection or low virulent organisms.
2. Artificial through vaccination.

Passive Immunization

This is the process of acquiring protective immunity without the need for an immune response on the part of the individual.

Passive immunity may be:

1. Natural transfer of Ab (IgG) from mother to infant takes place through placental circulation or colostrum, a rich source of IgA.
2. Artificial passive antibody therapy (serum therapy), e.g., administration of immunoglobulin.

MAJOR CYTOKINES

Interferons

Important CD Molecules