Introduction

Viruses are small particles which infect living cells; this makes them obligate intracellular parasites. They have no reproductive mechanisms of their own so instead must use host cells to replicate. There are two main threats for a virus; the host’s immunity and the death of the host. Both of which will typically prevent the virus from propagating.

Many viruses are able to survive within the host for a long time without causing disease to the primary host. However when this virus is transferred to a secondary host, it is possible for a lethal disease to arise as a result. (An example is the transmission of the rabies virus to man). Vaccinating against the harmful effects of a virus therefore works best in the secondary host where the virus is not well adapted.

Structure of a Virus

Particles of a virus are known as virions. Virions encapsulate the nucleic acid core, which is surrounded by a layer of lipoprotein (or protein); this is known as the capsid. The degree of complexity of a virus can differ greatly. Viruses can contain either RNA or DNA and they can be either single stranded or double stranded, any combination of these still allows the virus (once in the host cell) to replicate.

Viral Pathogenicity

The virion is able to bind to a wide range of molecules to mediate attachment and internalisation by a host cell (by endocytosis). Once inside the cell, the capsid of the virion breaks down releasing the viral nucleic acid into the cytoplasm of the host cell. Once inside the cytoplasm, the viral nucleic acid is able to replicate and at the same time, it is also able to inhibit the production of DNA/RNA (thus protein synthesis) of the host cell.

Possible outcomes for a cell infected by a virus:

• Lytic Infection – Host cell is destroyed, this is caused by virulent viruses

• Persistent Infection – Host cell is not lysed, but virions are released slowly, over a longer period

• Latent Infection – Occurs when there is a delay between the infection by the virus and the onset of symptoms

• Transformation – Some viruses are able to transform a normal cell into a tumour cell

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Introduction

Bacteria exist naturally on many biological surfaces, for example the skin or the lining of the intestines. Bacteria like these make up the body’s natural flora and have a range of symbiotic relationships; a good example would be the flora of the rumen in cattle which degrade food materials, providing energy for both the cattle and the bacteria. The three main types of symbiotic relationship are:

• Mutualism – Both members of the symbiotic relationship benefit
• Commensalism – No apparent harm/benefit occurs to either member of the relationship
• Parasitism – One member of the relationship is living at the expense of the other resulting in disease

The pathogenicity of a certain bacteria depends on its survival inside the host – how well is it able to resist or evade host defence mechanisms and immune response. The resulting disease/damage caused to tissue is due to either the pathogenicity of the bacteria or the immune response of the host itself.

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Introduction

There are two main types of immunisation, either passive or active immunity. Passive immunity being that derived naturally from a mother. A young animal will gain antibodies from its mother either during birth via the placenta or shortly after birth when the animal consumes colostrum from its mother. It is also possible to gain passive immunity by artificial means, this involves injecting an individual with an antisera (containing specific immunoglobulins). There are advantages and disadvantages of this artificial passive immunisation, the main benefit being that the resulting immunisation is immediately effective. The disadvantages include:

• Temporary effect, lasting only until the Ig proteins are metabolised (a few weeks)
• Only has an effect with diseases where an antibody response is the principle method of protection (as opposed to cell-mediated responses)
• There is the possibility that hypersensitivity may arise from use of serums which have been obtained from foreign species
• Induction of an active acquired immunity is blocked

The other type of immunisation is active immunisation, this is actively acquired immunity derived from either a natural infection or from artificial immunisation (inoculation with a weakened or dead organism).

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Introduction

The complement system or complement cascade as it is also known is a complex system of multiple proteins involved in inflammation and immunological response. The components of the complement system can be found throughout the body in fluids, providing the body with a systemic means of protection. Antibodies depend on complement for many of their biological activities.

Why is complement important?

• It opsonises pathogens to promote phagocytosis by phagocytes which display receptors for complement
• Certain components of complement act as good chemoattractants recruiting and activating phagocytes at the site of infection
• Complement structures can cause cytolysis or damage to certain bacteria by puncturing their membrane

The important protein components of complement are number C1 to C9 (they are numbered in their order of discovery however and not their order of action as you will see later). Upon activation certain components may split into sub components, usually the small components are named with an ‘a’ e.g. C5a (these are the components which are able to diffuse through tissue readily) and the larger components with a ‘b’ e.g. C5b (these are the components which do not easily diffuse).

The complement system is known as a cascade because of the triggering and amplification of further components of the system. In the cascade once a component has been activated by a proteinase, the molecule itself which was activated becomes a proteinase for the next component of the cascade. The whole complement cascade can be triggered in its entirety in a matter of microseconds. During the activation process the smaller ‘a’ subcomponent peptides which are formed mediate many of the other effects caused by the complement cascade, for example acting as chemoattractants.

There are three types of complement cascade, the classical and alternative pathways and the Mannan-binding lectin pathway. Both provide a path to the cleavage of C3 which is a central event in complement activation.